show comments and drafts count in changes browser

- for each file in the changes browser
- made ReviewCommentSink observable (to be notified of new or
  deleted/removed comments)

Closes: #80
add PathUtils class

- move getRelativePath to new utils class (multiple usages)
Merge pull request #86 from forrert/changes-browser/file-comment-count

show comments and drafts count in changes browser
implement possibility to select diff base in change viewer

- previous revisions or the change's base can be selected as
  a diff base
- comments in RevisionCommentSink are now linked to a
  revision as well as the change
- related to issue #22
Merge pull request #88 from forrert/changes-browser/diff-base

implement possibility to select diff base in change viewer
add commit message as a file to the changes browser

- similar to gerrit's web UI the commit message is now displayed as
  a file in the changes browser view
- comments can be viewed and added just like with other files
- resolves #87
better support for merge commits CommitMessageFormatter

- support commits with more than 2 parents
Merge pull request #89 from forrert/changes-browser/commit-message

add commit message as a file to the changes browser