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.
Hm, this comment is not really required, as it adds nothing on top of the automatically generated comment, no? But anyway, this is not an issue of this pull request - the pull request only improves the comment, so this is fine.