Extend parameter definition support in swagger 2.0

[wasbazi]

Handles the case described in #47 where a parameter can't be specified as a schema.

[coveralls]

Coverage remained the same at 93.75% when pulling b0142cd on wasbazi:parameter-definitions into 8810da3 on Yelp:master.

[coveralls]

Coverage increased (+0.04%) to 93.791% when pulling 283b8dd on wasbazi:parameter-definitions into 8810da3 on Yelp:master.

[sjaensch]

@wasbazi could you please add a test for your change, and a comment as to why you're copying the spec dict? It's not obvious what you're fixing, nor can we be sure that future changes won't break it again.