Add repo functions to omf utily

- Pull repository from origin unless upstream exist.

Based on
https://github.com/Homebrew/homebrew/blob/master/Library/Homebrew/cmd/update.rb#L175-L221

Extract _write_bundle to omf.persist

- Add support for persisting theme information.
- Use documented private function naming scheme.

Fix a typo: succesfully => successfully

Remove pkg-/theme- from folder name

When installing packages (plugins) or themes using a URL, we should
strip the plugin-/pkg-/theme- from the repository name if it exists.

Only send travis notifications when build fails

Add python plugin

Beginning of many PRs to clean up the code

This variable records the value of $PATH environment variable before
oh-my-fish is sourced. When we "reload" the framework, we reset $PATH
with this recorded value so that we boot from a clean state.

This was actually trying to load PATHs that do not exist under $OMF_CONFIG
directory and sourcing files that did not exist.

We don't use it for anything.

Add osx package

Make `default` theme the default theme for omf.

* On clean install, configure and download `default` theme.
* Do not allow `default` theme to be removed.
* When the current theme is removed, fallback to `default`.

Add gem package

Gives back a way to persist and update installed packages (dotfiles)