Add android build and fixed server SIGINT

The SIGINT was being ignored when the server only option was set. We now
gracefull close the server on a SIGINT

Add the -a --android option to build n android
Add feature to expire a file indevidually

You can force an expired file using `expired(true)`. It does not delete
the cache (not expunge()). It only expires it to force a new download
and change check.
Spelling fixes to file_loader documentation