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
TW-30472
Use resolved settings of GitHub build feature
Check if GitHub build feature was enabled
Merge branch 'master' into comments
fixes #27
disable url check for %references%
Update README.md
test for #20
fix test for #20
patched messages to expect trimming,
fixed #20
Merge branch 'fix20'
use ellipsis instead of '...'

#20
closes #26
cherry-picked README.md from vlad-experiments branch
reverted, added more logging,
log GH connection errors as WARN,
added debug logging for all requests
updates issue #38
added "teamcity.github.http.timeout" teamcity property to specify GH request timeout, default is 5 minutes
closes #37