fix #17: Change the attribute that provides the device groups list on the link elements read by BrowserMap

* the attribute where the device groups are defined for each site variant on the link elements is is now set to
  'data-bmap-devgroups'; the 'media' attribute is not analysed anymore
fix #18: Add BrowserMap version number as global variable to the BrowserMap object

  * added BrowserMap.VERSION variable;
  * created probe that returns this information