New cop Metrics/AbcSize

Measures the Assignment Branch Condition metric defined by http://c2.com/cgi/wiki?AbcMetric

`AbcSize` is a complement to `MethodLength`, which is also about method size, but is a "stupid" line number metric.

This is the result of a pair programming session between myself and @jfelchner. Thanks a lot for your contribution, Jeff! Any remaining mistakes are my own, though.