Add a settings package

This lets us modify the libgit2-wide options/settings.
Merge pull request #76 from jezell/walk-enhancements

Add missing walk functions
Adjust to Go tip changes

It does not like breaking aliasing rules, so let's keep a casted pointer
for when libgit2 wants that.
Merge pull request #75 from jezell/add-conflict-to-index

Add index conflict functions
Merge pull request #73 from libgit2/cmn/settings

Add a settings package
Merge branch 'master' of http://github.com/libgit2/git2go
rename inmemory to anonymous remote
Merge branch 'inmemory-to-anon'