when a build starts, mark GitHub status as pending
add configuration option for PR build comment

Added a configuration option of "add_build_summary" (boolean) that triggers whether the plugin adds a comment to the PR with summary information about the build.
Merge pull request #13 from mrkrstphr/build-summary-option

add configuration option for PR build comment
Merge pull request #12 from mrkrstphr/mark-build-pending

when a build starts, mark GitHub status as pending