Add Remote#Ls.

Allow filtering heads by name.
Merge branch 'master' into ls_remote

* master:
  Update libgit2 submodule.
  Remove Config#Refresh
  implemented Index.AddAll, Index.RemoveAll, Index.UpdateAll
Add connect methods to Remote.
Make filtering logic more simple.
Hide C.git_direction type.
Merge pull request #127 from calavera/ls_remote

Add Remote#Ls.
Make the constants have types

While Go will assign the correct type to a const block when it
auto-creates the values, assigning makes the const be typeless and will
only gain it in each particular use.

Make each constant in the blocks have an assigned type.