Change refspec api

New attempt to provide refspec API with pygit, now using the new functions in libgit 0.20

It will expose:

the attributes 

* fetch_refspecs
* push_refspecs

returning a list of strings with the refspecs

the methods

* set_fetch_refspecs([str])
* set_push_refspecs([str])

setting a list of strings a new refspecs