Fixed typo in comment.
Added Either-instance to Data.Types.Injective
Fixed type in comment in Data.Types.Injective.
Closes #40.

Replaced System.REPL.Command.sanitize with readArgs, which parses
parsec's stringLiterals.
Commands now correctly handle quotes.

Other (related) changes:
* Renamed ParamNumFailure to ParamFailure.
* commandDispatch prints an error message in case of mismatched quotes
(or other parsing errors).
* commandDispatch now has a monadIO instead of a Monad constraint.