Relax polisher deps, gracefully fail if missing (rev 4)

Now polisher gem contains minimal runtime deps w/ mechanism
to catch runtime load errors. Components missing reqs will
not be accessible / calling methods on them will indicate
to install optional dependencies

Rebased against latest master
Merge pull request #72 from ManageIQ/relax_deps

Relax polisher dependencies, gracefully fail if deps are missing
Fix #37: raise errors on build failures (rev 5)

Also handle a few edge cases / add some syntactic sugar.

Rebased against lastest master
Improve file detection / conversion during spec update

More edge cases are detected & handled though code could use
some cleaning up / refactoring and specs need to be written