Drop envy_ prefix from command module names
Automate loading of cloudenvy commands
Refactor subparser generation
Refactor command initialization

* All commands now inherit from a common baseclass: cloudenvy.envy.Command
* Each command object will have an attribute commands which is a dictionary
  tracking each initialized command object by module name
Fix typo s/Uplaod/Upload/
Merge pull request #107 from cloudenvy/refactor

Lots o' refactoring commits