buttonMarkup: Fix bugs that buttonMarkup doesn't set options properly and doesn't get a value of options.

Hi All,

I know that it will be deprecated, but I hope that my PR can help you improve/stabilize current ButtonMarkup.

**Tests :**
You can check issues to click three test buttons of bottom on the following test page:
* Origin(master): http://hyunsook.github.io/jqm-debug/latest/buttons-buttonMarkup_getter-setter_origin.html 

    Here are issues which were checked on that origin test page.

    * Getter: 
        - Sample Code: `btn_corners.buttonMarkup( "option", "corners" );`
        - Expected outcome: Retutn *true* or *false*.
        - Actual coutcome: No return *true* or *false*, but return *buttonMarkup own object*.

    * Setter: 
        - Sample Code: `btn_corners.buttonMarkup( "option", "corners", false );`
        - Expected outcome: Buttons is changed to square off corners.
        - Actual coutcome: Buttons is still round.

    The other options are same result with that.

**Solution :**
To fix the issues, my team modified jquery.mobile.buttonMarkup.js as follows:
1) Add isAlreadyEnhanced method to check if a button is enhanced.
2) Add codes to be able to function as setter or getter in according to options on buttonMarkup upper part.

You can see that the bugs are fixed on the following page:
* Modified: http://hyunsook.github.io/jqm-debug/latest/buttons-buttonMarkup_getter-setter_after.html 


If there are any problems or mistakes to go over it, please let me know.