Don't nil a Literal's units when its value is zero

I have no idea why I wrote it that way. It definitely doesn't make sense.
Renamed is_valid_unit? to valid_unit?

To make it more Ruby-like
Added a convenience method for creating bare Units objects

eg. Units.meters is the same as Units(:meters)
Fixed a small bit of silliness
Implemented simple unit conversions

It can only convert between inches and meters for now, but the basics are there. Additional conversion factors can be added as needed.