submitForm was sending value of buttons

Using submitForm, default values that were sent included all form
buttons.  Added the ability to select which button values should be sent
(emulating clicks on buttons and submit buttons)
Updated interface submitForm signature and docblock

Updated method signature for submitForm to include new $buttons argument
and modified documentation for docblock
Missed parameter in docblock
Change parameter name and documentation wording

Parameter name changed from $buttons to $button as it would normally be
used to set a single clicked button state.
Documentation changed from "I" pronoun to "You"
$button only accepts single value, WebDriver submitForm updated

Firefox and chrome don't post the value of a 'clicked' button to the
server when submitting a form, only the submit button's value is
posted, so no point in having an array.

Modified WebDriver module's submitForm to also accept a button
name which is 'clicked' if provided.
Merge pull request #1518 from zbateson/submitForm-buttons

submitForm was sending value of buttons