fix Mercurial tests and add tag validation

[krtek4]

This should fix #77

What do you think @jeanmonod ?

[jeanmonod]

I don't know if it's the right place to put this mercurial specific code... At first I was more expecting a new method on the VCS interface. Something like supportTaggingWithInteger(). But then the error have to be raised after the config have been parsed, at the beginning of the releaseaction...

[krtek4]

I wasn't really happy about this either.

Maybe a validateConfig on VcsInterface ?

[jeanmonod]

Why not a validateContext() on the version-generator?

[krtek4]

As discussed, because the generator work is a bit different than the other actions and the persister and there's no need at the time.
Adding it once we need it won't be a big issue.

Can we merge ? please, pretty please, PLEASE ! PLEASE !

[jeanmonod]

There is two last issues, my small comment about the prerequistes in the list and the compatibility to 5.3, (see the Travis builds). Except that, I guess we can merge! Thanks for your work

[sseidelmann]

@jeanmonod Whats with this PR? We should integrate the HG Tag check. All my HG tests are failing because of this.

[dbu]

doing a minimal fix of the tests without the full improvement and better error reporting in #130