Add pay_invoice and allow new card with post_card.

Add new pay_invoice method, and fix a bug with post_card not
recognizing a new credit card rather than an update.
Add documentation of pay_invoice.
Bump to version 0.17 start using PkgVersion plugin
Change version string in access log
Merge pull request #29 from rustyconover/master

Version 0.17
Allow trial_end to be both an integer and a string.  Support trials ending "now".

Bump to 0.18
Merge pull request #31 from rustyconover/master

Allow trial_end to be both an integer and a string.  Support trials ending now.  Release 0.18.
Fix list_subscriptions method, add tests for list_subscriptions