add to keyWindow instead of rootVC.view

It causes weird behavior when view is scrolled.
change interface of WPYPaymentViewController

- receive price tag parameter to show as price label
- subclass from UITableViewController to handle
scroll when keyboard is displayed.
add price view to WPYPaymentViewController

To dismiss keyboard when touched outside of textfield,
the controller needed pointers to textfield.
The controller now creates WPYAbstractField subclasses
own its own, and does not use WPYCardFormView any more.
[design] change price tag color to wet asphalt &
form label color to iOS standard blue.
adjust WPYItemDetailViewController to interface 
change in WPYPaymentViewController
update README.md to follow new interface of 
WPYPaymentViewController
use spacedVisaNumber & hyphenedVisaNumber instead
of string literal.
use static variable validName instead of string literal
add explanation that the numbers used are for test
purpose.
validate public key before creating token

restrict use of invalid public key.
use constant string instead of string literal in test
remove unnecessary TRVSMonitor in WPYTokenCreationTest

Trying to create token with invalid card will
return before making actual http request, so waiting
for async code is not necessary.
[design] adjust font of cell label & size of price
view