Added typeclass for uninhabited types

This typeclass can make pleasing the completion checker much easier, as well
as when using a contradiction.
Merge pull request #225 from david-christiansen/uninhabited

Added typeclass for uninhabited types
Complete identifiers following non-whitespace

Now, identifiers following non-whitespace, non-identifier characters (such as
operators or parentheses) will also be completed successfully.
Fix comedy error in termination checker
Only use metavariables as completions for :p and :prove
Stop completion of names that are prefixes of others

Previously, when completing "plus", it was accepted. Now, it tries completing
with "plusAssociative", "plusCommutative", etc, and the user has to press
space.
Merge pull request #228 from david-christiansen/completion

Completion usability improvements
Add myself to CONTRIBUTORS

(gotta stoke the flames of ego)
Merge branch 'master' of github.com:david-christiansen/Idris-dev