use vcstool wrapper of urlopen using netrc

For review (should be quick) and merge after this one:
https://github.com/vcstools/vcstools/pull/60

This should make it possible to merge with rosinstalls lying on bitbucket in a private repo, using netrc credentials to access the url.
E.g. 
    
    ../scripts/rosws merge https://bitbucket.org/tkruse/rosinstalls/master/raw/test.rosinstall

should be possible if there is a file ~/.netrc with contents like

    machine bitbucket.org login <user> password <password>