Added support for mobile capture attribute

I added an option that will allow using the capture attribute as defined and outlined by w3c (see section 5 of http://www.w3.org/TR/html-media-capture/).  Mobile browsers implement this specification with subtle differences from what the w3c outlines (see http://mobilehtml5.org/ts/?id=23).

It is impossible to write tests that would test this feature in the current test suite, but the current tests all pass.  I have tested the implementation on Chrome and "Internet" on Samsung Galaxy S5 and Samsung Note 3.  I also tested the implementation on Safari on iOS 7 the iPhone 5S.  The attribute is ignored in older browsers that do not support the capture attribute.