Add functions for PATH prepends/appends/removes

* path_append appends a dir to the end of PATH
* path_prepend prepends a dir to everything else PATH
* path_remove removes a dir from PATH
Only activate Cordova with cordova-activate/deactivate functions
Clean up .zshrc quite a bit

* Change ENVPATH to DOTFILES_PATH
* Change all PATH munging to use path_* methods
* Remove duplicate rvm load block
* Merge everything from ~/dx into ~/app
* Export FINK_HOME, PYTHONBREW_HOME for consistency
* Minor formatting fixes for readability