implemented Index.AddAll, Index.RemoveAll, Index.UpdateAll
Remove Config#Refresh

Because it has been removed from libgit2:
https://github.com/libgit2/libgit2/commit/55cb4999720f46eddb97e25e7ddc6ac068dd0bae
Add Remote#Ls.

Allow filtering heads by name.
Update libgit2 submodule.
Merge pull request #128 from calavera/remote_git_config_refresh_call

Remove Config#Refresh
Merge pull request #125 from lucas-clemente/master

implemented Index.AddAll as git_index_add_all wrapper
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.
Merge pull request #129 from libgit2/cmn/const-type

Make the constants have types
Updates README.md to include GoDoc badge.
Merge pull request #132 from apsdehal/master

Updates README.md to include GoDoc badge.
README: add -d flag to 'go get'

To make sure it doesn't try to build it.
Fix MergeTrees func to accept nil as ancestor parameter
Merge pull request #134 from joseferminj/fix-merge-trees-nil-ancestor

Fix MergeTrees func to accept nil as ancestor parameter
Expose DiffTreeToWorkdir function
Merge pull request #135 from joseferminj/diff-tree-workdir

Expose DiffTreeToWorkdir function