Add option to not rebuild commits that already have a status

Replace the BUILD_ALL_COMMITS setting with a BUILD_COMMITS setting that
can be one of 'ALL', 'LAST', or 'NEW' that build all commits, only the
last commit, or only commits without a status.

This setting defaults to 'NEW'.

This is backwards compatible with the previous BUILD_ALL_COMMITS option.
BUILD_ALL_COMMITS is translated to BUILD_COMMITS as follows:

    BUILD_ALL_COMMITS = True   <==>  BUILD_COMMITS = 'ALL'
    BUILD_ALL_COMMITS = False  <==>  BUILD_COMMITS = 'LAST'
Merge pull request #20 from rouge8/build-only-new-commits

Don't rebuild commits that already have a commit status