fix for getting scm url and branch for hg repos

Hi, 

I have made these changes to address the issue that 'get_scm_url' is not returning 
the correct url for hg repos [1].

I also noticed that the mercurial plugin doesn't store the branch name in 
config.xml corresponding to the job when the branch used for build is
'default'. I have modified the code to handle this scenario.

1. https://github.com/salimfadhley/jenkinsapi/issues/199