Parameter.required flag implemented

Made the "required" flag for Parameter objects available to the code generator templates
Merge pull request #310 from martoe/param_required

Parameter.required flag implemented
added required flag for parameter
Merge branch 'develop_2.0' of github.com:wordnik/swagger-codegen into develop_2.0
fixed build error
changed to Boolean to allow null values
added model pacakge
fixed summary/description mapping
fixed import types
MCA-specific extension
Make response headers available to the code generator (tryout)
"External Documentation Object" implemented (according to spec)

Depends on https://github.com/wordnik/swagger-core/pull/716
MCA-specific extension

* remember last template execution context (to generate an output based on multiple specs)
* don't swallow exceptions