add support for file comments on both sides (parent and revision)
set file review status to reviewed when opening files in diff viewer

- review status is not visible in plugin yet (only web ui)
- review status cannot be changed in plugin yet

Related to #20
add note to error message that start feature is only supported in Gerrit >= 2.8
bump version to v0.5.0

- update changelog
Merge remote-tracking branch 'origin/master' into intellij13