Fixes and cleanups.

Add default card parameter to post_customer().

Fix _post to handle objects being included in argument, convert them
to form fields if they are nested inside of a posted argument hash.

Add id to the Card object.

Add cards to the Customer object as a Net::Stripe::List.
Bump version to 0.15.

Add keep alive to user agent.
Move from MooseX::Method::Signatures to Kavorka
Bump to 0.16
Merge pull request #27 from rustyconover/master

Version 0.16