Intelligent handling of "extra" slashes in pow/tgh

Gatherer formats fractions using slashes and curly braces, and these
sometimes appear in power/toughness stats.  The extra slash or slashes
causes a failure with splitting these numbers apart.

The solution here a regular expression to lookahead for slashes that
are either followed by a fully enclosed curly bracket string or no
brackets.  Split the power and toughness only on those slashes.
Merge branch 'master' of github.com:chigby/mtg