model: remove BaseModel class

The only remaining purpose of this class was to provide the "sa" field,
allowing a custom SQLAlchemy session to be used for model operations.
However, this never actually worked, nor was it used anywhere.

There's always a global SQLAlchemy session associated with the current
thread; using another session for a single function call does not make
any sense (as sessions cannot be mixed), unless the code works carefully
to ensure the two sessions (and all objects loaded from them) are kept
completely separate. Suffice to say that Kallithea does no such thing,
thus there's no need to pretend to support multiple concurrent sessions.

model: move code from __init__.py to base.py

Having too much code, in particular too much imports, inside a package's
__init__.py is a recipe for circular imports, and considered bad practice in
Python [1]

Move out everything from kallithea/model/__init__.py to a new file
kallithea/model/base.py and adapt the existing imports.

[1] http://docs.python-guide.org/en/latest/writing/structure/#packages

model: drop BaseModel.cls

It no longer serves a purpose.

remove vestiges of Python 2.5 support

We only support Python 2.6 and 2.7; hence we do not need to import
with-statement support from __future__.

cleanup: check for None object identity in cases where that is what the 'contract' says

Avoid applying bool() on complex objects - it might return something unexpected
such as the key (which might be 0 and thus be false). Checking for None is more
safe and faster.

utils: make API key generator more random

The API key generator abused temporary filenames in what seems to be an attempt
of creating keys that unambiguously specified the user and thus were unique
across users. A final hashing did however remove that property.

More importantly, tempfile is not documented to use secure random numbers ...
and it only uses 6 characters, giving approximately 36 bits of entropy.

Instead, use the cryptographically secure os.urandom directly to generate keys
with the same length but with the full 160 bits of entropy.

Reported and fixed by Andrew Bartlett.

cleanup: avoid some 'except Exception' catching - catch specific exceptions or log it and show what happened

This has a risk of introducing regressions ... but we want to get rid of all
exception muting and make the whole system less fragile and easier to debug.

Correct licensing information in individual files.

The top-level license file is now LICENSE.md.

Also, in various places where there should have been joint copyright holders
listed, a single copyright holder was listed. It does not appear easy to add
a link to a large list of copyright holders in these places, so it simply
refers to the fact that various authors hold copyright.

In future, if an easy method is discovered to link to a list from those
places, we should do so.

Finally, text is added to LICENSE.md to point to where the full list of
copyright holders is, and that Kallithea as a whole is GPLv3'd.

Second step in two-part process to rename directories.
This is the actual directory rename.