Support for async membership providers

[lafe]

Proposal for #34 that adds the possibility to add asynchronous membership providers. Two new interfaces have been added:

• A new base membership provider that is the base for both (sync and async) membership providers (so that registering it in the IoC container remains basically the same)
• A new IAsyncMembershipProvider that offers an async endpoint

The PassCommandHandler checks the type of membership provider and performs either a synchronous or an asynchronous call to validate username and password.

[fubar-coder]

I'm hesitant to add this yet, because I'd just change the IMembershipProvider member to async and bump the major version.

[fubar-coder]

Closing in favor of async IMembershipProvider.