Add git2r_repository_discover to wrap git_repository_discover

* takes a starting path, looks for repository path
* does not cross filesystems
* does not support ceiling_dirs argument
New workdir "character" method returns git workdir for path
make sure that workdir(character) has trailing filesep

* like workdir(repository)
tests for workdir(character)
Cleanup/error handling/return to standard style

* note that this now errors in cases where there
  would have been a nil return.
Update tests to reflect change in behaviour
add repository_discover generic/character method
don’t error out if we can’t find the repo

* but there was not a more serious error
fix tests for new behaviour
rename repository_discover  function to discover_repository

* better encapsulates the action
* see also http://www.pygit2.org/repository.html
remove workdir character method

* discover_repository essentially replaces this
  although one may need to do:

  workdir(repository(discover_repository(path)))
* also remove tests
Merge pull request #36 from jefferis/feature/workdir-for-path

Feature/workdir for path