Merge pull request #170 from nvloff/ruby_2_travis

add ruby 2.0.0 to travis config
bump libgit2 to 15a63e
bump libgit2 to 677dce
bump libgit2 to 50eb85
bump libgit2 to 5b27bf
bump libgit2 to 7dbf40
bump libgit2 to 33a594
remote: add docs
remote: add remote.add
remote: add tests for transport methods

This doesn't actually test the transport protocols, just that
calling the methods transporting data actually works.

File transport is used so, that the tests can be faster and work
without network connection
remote: add remote.url=
remote: extract the check for valid url