Added design doc
Add utility functions

* gitUtils.go contains methods for getting information about
  the current git repo
* commandUtils.go contains a method Execute to execute a system
  command
Add package config with methods for configuring sgit

* Setting projectType doesn't work yet
Add packages cache, git, main

* cache contains methods for saving and restoring the cache
  on certain git commands
* git contains methods for running git commands and using the
  cache when appropriate
* main runs sgit
Add gitignore

* Ignores .swp files
Clean up command-line flag retrieval

* still doesn't retrieve flag value
Add file for errors in package cache
Clean up errors