be more inclusive while unregistering keys in uninstall. Wipe out both 32 and 64 xll's OPEN HKCU keys.

Add comment about a tricky situation where user may have change of Office version (32 to 64 or conversly)
missing namespace for commit 0c146e8b075cdbf30a9ea20602d6e1be0a491df4
Merge pull request #5 from bpatra/master

Potential troubles for uninstalling an addin if Office bitness has changed + fixup pullreques#4