Remove :STRING on EXTRA_REPOSITORIES var

You don't need to set the type on cache vars with CMake anymore as long as the
default type is correct.
Use git describe for git version in doc files

Now the version for these files will automatically get generated using 'git
describe'.  Therefore, you just need a nice clean tag and then you get:

   <tag_name>-<num_commits_after>-<sha1>

That provides a way to uniquely identify the TriBITS version.