Added tab-completion in inferior-mongo shell.

Most of the code was borrowed from python.el, No changes were made to
existing functions except for the function 'inf-mongo'.
Set inf-mongo-command, inf-mongo-buffer only when a new mongo process is started

Also setup auto-completion only when a new mongo process is started.
Fix typos
Merge pull request #4 from iqbalansari/master

Added tab-completion in inferior-mongo shell