Move distribution pom.xml to root

This is a change to distribution assembly archive layout.  In the old
archives, the maven test suite failed because the data directory
wasn't in mvn's working directory.  Apparently at least one test
requires stuff from the data dir.  My changes fix the path problem.
Make gollum-site-wrapper check for detached HEAD

Gollum dies with a cryptic stacktrace when its run on a wiki repo with
a detached HEAD.  Wiki repos should always be on master anyway.  Now
the wrapper checks that the doc repo is on refs/branch/master and
exits with an error message otherwise.