Fixing specs

Hi,

I spent a few hours fixing some of the stuff that seemed obviously wrong. Some of it was Ruby 1.9.3 compatibility, some of it was broken/inconsistent specs, and some of it was (apparently) bugs in Quantity itself.

Most of the low-hanging fruit is fixed, but I'm still not clear what's wrong with the rest of the specs, or how to fix them. Once there's a Travis build I'll point out some specific problems for us to discuss.