Better fraction recognition

#### I wanted to use your gem in a project, but I had a couple issues:
1. The recognition of fractions was skimpy and would accept any string with a forward slash which is easily confused
2. There is no support for negative denominators for single fractions. (Note* that I do not think this should be possible for mixed fractions)
ex:   1/-2 should be acceptable but 1 1/-2  should not be acceptable

#### Changes
To fix the above I have added some class variables to hold the regular expressions that are used to parse mixed and single fractions.  These will be used in both the testing functions (i.e. fractional? single_fraction?  mixed_fraction?) as well as in the to_f method for parsing fractions.

Let me know if you would like these two issues separated, or the commits squashed a bit.