Support for providing the committer (in addition to author) in the API

Users have a requirement to distinguish between the author and the committer in pre-receive hooks etc.

Although this is possible to get the committer information via the low-level git API, it's much more cumbersome than the higher-level API which returns for example com.atlassian.bitbucket.commit.Commit objects.

Consider adding the following methods to Commit:

@Nonnull
Person getCommitter();

@Nonnull
Date getCommitterTimestamp();