Fixes http://github.com/SAP/openui5/issues/207

Added the ariaDescribedBy and ariaLabelledBy associations to the
sap.m.Button control. Also updated the explored page for buttons to use
the new associations. I added a new disabled button to the explored sample
to ensure that bringing over the code from the desktop version of Button,
which used a slightly different handling of the disabled property, still
worked correctly.

In addition to the functional changes I made some minor changes to eliminate problems flagged by your ESLint process. Since your contribution guidelines include a clean run of ESLint as a prerequisite I figured this was the right thing to do, but if I should ignore such warnings in existing code please let me know.

Note that I already have a signed corporate contributor agreement on file with SAP.
thanks for also adding the tabindex, i saw it was missing before (using disabled attribute)