Attach the sources during packaging

Since this project is Open Source, it is very handy to have Maven
download the sources for the jars found in central.

Some projects also attach javadoc, but the sources provide javadoc
(at least with IJ) so I find that they just take up space.
Refactor to InputStream instead of URL

InputStream is the common denominator and contains enough information to
construct the ThreadBuffer (and thus the Reader).
Remove unnecessary throws declarations

Those warnings in IJ hide potentially real bugs.