Add option to add comments to pull requests

 * add comment for every build stage to github pull request
 * you can enable this functionality it as part of plugin config
Merge pull request #24 from ctomc/master

Add option to add comments to pull requests with more details from TeamCity's build(s)
cleanup code,
add @NotNulls
fix obvious bugs
use resolved commit hash to post comment,
avoid possible NPE
polish UI
fix layout for UI
cleanup code, make it work
Merge branch 'master' into comments