Add a documentation directory.
publish-artifacts.sh: fix doc directory name.
Optimizations on FastLeaderElection.

Do not discard PROPOSED_EPOCH message during election.
Fix netty transport.

1. The server who clears the transport shouldn't get the DISCONNECT message.
2. Fix the null pointer exception.
Limit the size of the pending requests.
Merge pull request #176 from EasonLiao/fix_0.2.0

Fix for 0.2.0