DMAP-34 - Add a current variant attribute to the site variants BrowserMap uses for forwarding clients

* added BrowserMap.getCurrentVariant() which extracts the variant that has a ‘data-bmap-currentvar’ attribute with its value set to ‘true’
* updated tests
* bumped version to 1.4.0-dev (API changes)

git-svn-id: https://svn.apache.org/repos/asf/incubator/devicemap/trunk/browsermap/trunk@1543039 13f79535-47bb-0310-9956-ffa450edef68