add "(beta)" to download link
 * thanks to myrdd at https://github.com/RequestPolicyContinued/requestpolicy/issues/445#issuecomment-59946581
Download link directly leads to XPI
set version to 1.0.beta8 in update-1.x-beta.rdf
update download link for v1.0.beta8
version change to 1.0.beta8.1
add warning about 0.5x compatibility issues
 * closes https://github.com/RequestPolicyContinued/requestpolicy/issues/515