Added template tag to expose the Stripe public key.

[freakboy3742]

This adds an easy way to get the Stripe public key (the PINAX_STRIPE_PUBLIC_KEY settings) into a template so it can be used to configure the Stripe Javascript library.

[coveralls]

Coverage remained the same at 100.0% when pulling e9d8dc7 on jambonsw:templatetags into aaa4516 on pinax:master.

[coveralls]

Coverage remained the same at 100.0% when pulling e9d8dc7 on jambonsw:templatetags into aaa4516 on pinax:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 17e5d23 on jambonsw:templatetags into aaa4516 on pinax:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 17e5d23 on jambonsw:templatetags into aaa4516 on pinax:master.

[freakboy3742]

This patch includes the content of #297, as the test suite won't pass without it.

[coveralls]

Coverage remained the same at 100.0% when pulling a0de07e on jambonsw:templatetags into aaa4516 on pinax:master.

[paltman]

Thanks @freakboy3742!