added function for validating model definition modules
dvcs.latest_commit takes only path; can return commit for repo file

With this change only a path argument is accepted, no more of that
wishy-washy repo_or_path stuff.
If the path represents a file rather than a directory (and if the
file is actually inside a repo) the latest commit for that file
is returned.
valid models now have FIELDS instead of (COL*,ENT*,FILE)_FIELDS

Makes things just a bit simpler if we don't have to construct
a *_FIELDS var all the time.
bumped version number to 0.9.1
oops bumped version number on the wrong branch...
rm mappings.json (moved to 'ddr' repo)
rm facets JSON files (moved to 'ddr' repo)

Model definitions, facets, topic vocab, etc, etc are all
specific to the Repository rather than to the application
and thus should live in the 'ddr' repo rather than in the source.
rm model definition files (moved to 'ddr' repo)

Model definitions, facets, topic vocab, etc, etc are all
specific to the Repository rather than to the application
and thus should live in the 'ddr' repo rather than in the source.
function for comparing commits (newer,older,equal) in a branch

Indicates whether a commit is newer than/older than/equal to
another commit in the same branch of a repository.
Merge branch 'repo-models' into develop
function for getting path to module source file (.py not .pyc)