Create functions from the example form to make api more abstract

A bunch of the comments in stripe.admin.inc module expressed a desire to abstract out some of the necessary elements of the Stripe process. This patch makes a number of changes:

It adds two new functions in stripe.module that take care of the necessary additions to $form:

/**
 * Returns the stripe pubkey.
 */
function stripe_get_pubkey()

/**
 * Returns an array to add to $form['#attached']['js'].
 *
 * @param $pubkey
 *   A Stripe pubkey returned from stripe_get_pubkey().
 * @param $selector
 *   A jQuery (xpath) selector to the <form> tag.
 */
function stripe_attached_js($pubkey, $selector = '.stripe-form')

I made a number of changes to stripe.js so that it works with any generic form that matches $selector (rather than just working for #stripe-admin-test).

It also adds a generic function to process transactions with some basic error logic:

/**
 * Process a stripe transaction.
 *
 * This takes into account all submissions to this webform. The output of this
 * function will be displayed under the "Results" tab then "Analysis".
 *
 * @param $amount
 *   The transaction amount, in dollars (if the currency is usd).
 * @param $token
 *   The Stripe token from the stripe.js call.
 * @param $description
 *   The transaction description for Stripe.
 * @param $error_messages
 *   BOOL.  Should we print error messages"
 * @return
 *   The stripe response, or FALSE if there was an error.
 */
function stripe_transaction($amount, $token, $currency = 'usd', $description = NULL, $error_messages = TRUE)

I used the changes from this patch to build the Stripe Webform module. You can see in work with the changes to stripe.admin.inc in this patch.