Merge pull request #111 from boudewijn-tribler/bugfixes

Reduce pylint warnings.
Removed Constructor class.

BloomFilter remains the same, but figures its parameters out by
itself.  This simplifies the code and allows pylint to work correctly.
Disable incorrect pylint warnings.
Replaces .get_bits_checked() with .bits_checked.
Removed old debug code from crypto.py

Long ago we did experiments with crypto disabled (by hardcoding 'if
True' into crypto.py).  To my knowledge this has not been used since,
hence this hack has been removed.
sha1 was no longer used.
Merge pull request #113 from boudewijn-tribler/crypto-cleanup

Removed old debug code from crypto.py
missing-identity performance improvement.

Handling missing-identity messages was using an expensive sql query.
This has been replaced with two simple queries resulting in a
considerable performance improvement (all exising indexes can now be
properly used by sqlite).
New unittest and some cleanup.
Merge pull request #115 from boudewijn-tribler/bugfixes

Using Python's abc to handle abstract methods and properties.
Merge pull request #114 from boudewijn-tribler/performance

missing-identity performance improvement.
Merge pull request #112 from boudewijn-tribler/bloomfilter-cleanup

Removed Constructor class.
Merge pull request #116 from boudewijn-tribler/bugfixes

Bugfixes
Dispersy.stop will now return either True or False.

Previously Dispersy.stop would always return True.  Now the return value
depends on the various elements that are stopped/closed, i.e. endpoint,
database, and callback.
Merge pull request #117 from boudewijn-tribler/dispersy_stop

Dispersy.stop will now return either True or False.