Subtree merge in Git-Tfs wiki repo

Resolves #383
Fix links to wiki.
mv config.md into the one doc dir
Move the old home wiki page so that it's the default file in the doc dir.
Updated links.

* All file names are lowercase.
* All command docs are in the `commands/` directory.
* A couple of useless files are gone.
contributing update.
update libgit2sharp to v0.12
New libgit2 naming conventions

See https://github.com/libgit2/libgit2sharp/commit/96df5f8d6d3e35f8aadafdc25a95ede3eb9e86b5
Merge pull request #385 from sc68cal/docs and spraints/docs

Move docs from the wiki to the repo.
Manage rcheckin in bare repository
if another commit was done in TFS, keep the repository in good state before reporting error
prevent rcheckin by error in a bare repository
Modify fetch command to manage bare repositories
Manage clone and init in a bare repository
Add branch support in fetch and rcheckin for a bare repository

But should specify the git branch name that should be updated with --bare option

Use :
git tfs fetch --bare=master
git tfs fetch --bare=branch1 -i branch1
git tfs rcheckin --bare=master
git tfs rcheckin --bare=branch1 -i branch1
refactoring of how get ref for a head branch...
corrections....
correct indentation introduced by merge with master
Add a reference to checkin parameter to the checkin method!
fetch : throw error when ahead commit and introduce --force flag

return an error that fetch is not allowed when there is ahead commits in bare repository
and introduce a flag --force which, when used, do fetch anyway and loose the ahead commits