Move get_git_revision() to git.py
Add LEMON_DIR module-level variable
Add git_update()
Add --update option
Update usage message
Add --update option

Although fetching and merging the remote GitHub repository is as easy
as chdir'ing to the LEMON installation directory and doing `git pull`,
astronomers do not necessarily have to be fluent in git. This is just
a convenience function so that our end users can easily pull upstream
changes from the remote repository.

This merges branch 'add-update-option'