Support api versioning

[woylie]

Right now, BlueBird doesn't support api versioning. Everything is put into one single documentation. However, a version must be specified in Swagger. I suggest to gradually add support for api versioning.

1. If nothing else is specified in the configuration, use either "1" or the current mix project version as the api version.
2. Additionally, allow to specify a single global version in the config file.
3. Additionally, allow to configure BlueBird to use the base path for versioning.
4. Additionally, allow to configure BlueBird to use the accept header for versioning.

For 3 and 4, generate separate documentation files for each version.