Add gitdata create endpoints

[rvlander]

Adds the following features:

• Adds function to get commit using auth
• Adds endpoint to create new tree
• Adds endpoint to create new commit

[phadej]

This doesn't re-export new *R requests from GitHub module.

[rvlander]

I am unsure of what you are asking.

[jhrcek]

@rvlander he probably means that you should export the new functions ending in R (createCommitR and createTreeR) which you added and reexport them from the Github module.

[andreasabel]

@rvlander Is there still interest in this PR?