Add scripts to build and use a static libgit2
Switch default to static linking

Build in libgit2 statically into git2go by default, removing the need
for the right version to be available as a shared object.

We do still need to link dynamically against OpenSSL and LibSSH2.
Move some logic into the Makefile

This should provide a nice compromise between the scripts and comon
commands.
Use a submodule for libgit2

This makes building the static version easier as we know where the repo
is and that it's has a known-good version.
Adjust travis script to static
hack for travis
Really build statically
Move the settings into the main git2go
Tighten up the build

Build as release, and there is no need to install the library, we know
where the files are.
Don't build with Go 1.0

Its cgo linker doesn't support statically linking C libraries.
Upate README with static instructions
Remove left-over submodule configuration
add support for annotated tags
fix indentation
Update vendored libgit2 to v0.21.0
Merge commit 'refs/pull/95/head' of github.com:libgit2/git2go