Autocomplete

This PR adds autocomplete functionality to the command line interface:

* Added beginning of tab-complete framework for tab completing different data points from Jira
* Added caching framework for caching data returned from Jira
* Added tab complete functionality for assigning users
* Added assigning of a user when creating a new issue

NOTE:  I was unable to figure out how to do good tab completion from the command-line.  This is because it is very much tied to the shell you're using.  I would like to add tab completion for the assign user task, but this held me up.