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)

Add rvm package

themes: make 'agnoster copy' back into 'agnoster'

registry: add pbcopy plugin

describe without ruby

no dependency on ruby & a little quicker. all credit to @derekstavis for the
cool regexs

describe works on installed packages

also modified list_db_packages to return all packages by default, but with a
skip_installed to skip listing installed packages