Introduce SwaggerClientFactory

`SwaggerClientFactory` instance maintains the state of all swagger
clients and their timeouts. `factory` is a global instance of
`SwaggerClientFactory`, which is instantiated by the public factory
method `get_client(..)` if it is None, else it is used again.

`SwaggerClientFactory()(..).client` can be used directly if global
module variable 'factory' is not desired.
On second thought should these be made instance methods? They dont use the state of the class in any way. (the params still have to be passed)