Moving checking for/setting API key to startup method.
Adding createCustomer method & tests, with help from @louisroy.

The createCustomer method is partly based on pull request #3, but
with some additional changes and improvements.
Adding delete customer to test.
Updating createCustomer and formatResult, adding more tests.
Adding customer subscription (plans) support and tests.
Renaming createCustomer to customerCreate.
Adding customeRretrieve method and tests.
Adding Stripe_CardError exception catch to createCustomer.
Moving mode check before key check.
Allowing customerCreate to accept all API options.
Updating docs for version 2.0.