More flexibility in parsing Maven versions.

This change allows more flexibility in parsing Maven version that differ from OSGi convention, for example:
"1.2.0rc1" => Version(1, 2, 0, "rc1")
"1.2.rc1" => Version(1, 2, 0, "rc1")

Please check the included test case, if the altered behavior is fine. Parsing could be relaxed further to parse
"1.2.3.4.5" as Version(1, 2, 3, "4.5") but I'm not sure if this is desirable.
 
This PR is based on https://github.com/bndtools/bnd/pull/636, let me know if you prefer it squashed or rebased.