Convert all Symbol... names to Model...
Merge pull request #252 from apiaryio/pksunkara/symbol-to-model

Convert all Symbol... names to Model...
* Fix issue#158
* change Fixtures for integration tests - change header names according
to token specification http://tools.ietf.org/html/rfc7230#section-3.2.6
* define new WarningCode HTTPWarning
* refactoring - move validation out from SectionProcessor<Headers> into set of Functors
* additonal test with non-valid header name
* validate header name against token stecification
* split cases into mode TEST_CASE (by topic)
* add test for missng header value
* parse header with empty value