Create a Mock Framework

[whitlockjc]

We need to break the existing mock support out into a configurable part of swagger-tools. This mock framework will allow the user to register their own functions to generate mock values and to generate their own response structures based on acceptable mime-types.

This issue was originally discussed in #30.

[whitlockjc]

We also need to make sure we add the mocking of header values as a part of this as this is currently not available in swagger-tools' mock support.