Extend Functionality, Check Functionality, Reentrant Lock Changes

[bnbalsamo]

Implements the following new functionality:

• Tests for python 3.5 and 3.6
• Relevant boolean return value for RedLock.release_node()
• Corresponding boolean return value for Redlock.release()
• RedLock.check(): Checks to see if a lock is still held, returning a boolean
• RedLock.check_times(): Checks to see if a lock is still held, and returns each nodes reported TTL for the lock
• RedLock.extend(): Tries to extend the TTL of a lock back up to its original TTL
• RedLock.extend_with_validity(): Same as above, also returning the new lock TTL
• ReentrantRedLock.acquire_and_extend(): Acquires and extends (or just extends) a reentrant lock, incrementing the acquired counter.

Changes the following functionality:

• ReentrantRedLocks now respect lock timeouts

Added some doc strings and comments here and there.

Tests were altered because of the changes to ReentrantRedLock - in order to
test the functionality the tests now required a real lock, rather than a
mock. Tests were also added for new functionality.

[joaomoa]

Is there a chance we can get this merged in? We have been using this branch at Smarkets and find the functionality quite useful.

We use the redlock to elect master from a pool of instances and the master keeps extending the lock.