Rename functions with prefix gitr_ to match R package development conventions #18

Current trend is to begin function names with the package name + underscore, e.g. stringr::str_*()

This would be nice (and simple) to implement, and avoid the ambiguity of the git_*() and get_*()
that currently exists.

Make it uniform, clean, and easy to guess. This will involve a LOT of breaking changes,
but since we're at a 0-point version release, these things are expected to change often.