Update documentation to always mention the `store` option.
Raise an error when the middleware is misconfigured with unsupported options.
Emit a warning when using the MemoryStore as the cache backend.

This work perfectly for standalone processes and clients, but fails
badly inside webapps as the cache gets recreated every time the
client might be instantiated (on each request, for instance).
Update documentation
Merge pull request #26 from plataformatec/lm-safeguards

Add some safeguards to warn misconfigured clients