Prefix handling updates from olbrich/ruby-units#98

1. Throw "Unit not recognized" error on invalid prefixes (such as "mmm").
2. Add "mc" as alternative prefix for "micro" (apparently used in medical abbreviations such as mcg -- microgram).

These changes do not appear to break any existing unit tests.

Added new unit tests for the new functionality.

See: https://github.com/olbrich/ruby-units/pull/98