Better support of local html file + string in APK

HTML.LOAD.STRING from an APK now links correctly to the /assets base
drive to find Html resources
Opening a link to an anchor in a local html file from APK now works
correctly
See details in https://github.com/RFO-BASIC/Basic/issues/49
Merge pull request #65 from mougino/master

Html support in APK (issue #64)