Permit toggling of various checks via property file (assertj-swagger.properties)

[benfowler]

This pull request is a first attempt to give assertj-swagger users a little bit of finer control on what assertions are tested. Changes outlined below.

@RobWin : happy to rework this as needed, to get this into sufficiently good state to merge. Looking, especially, feedback on whether you think I've gone about making these changes sensibly, and what could be improved.

• Some simple toggles are added to turn various types of checks on and off (see SwaggerAssertionType). This is to work around imperfect normalization of definition attribute type checks that are referenced through $ref (however, some groundwork has been laid to rectify the latter)
• Added a mechanism to resolve consumes and produces values (AttributeResolver inner class); this can be extended later to normalize type references through $ref, etc
• Added a means to whitelist paths and definitions which are missing from the 'actual' schema -- this is to permit validating against a contract-first schema, where only some of the endpoints have actually been implemented.

[RobWin]

Thx for your effort. I will definitely merge it. Could you please add a documentation of it in the README?

[benfowler]

@RobWin: cheers!

• I'll get to the README shortly and then ask you afterards to take another look at this PR.
• In a separate PR, I'd like to rework definition attribute type validation to normalize the type definitions (by traversing local $refs). Do you have any thoughts on what you'd like to see done to make this useful and effective?