Update README to point to GitHub

Incremented version number to 3.0.21 in preparation for next release.
int: and ring: now respect trace_interrupt

Calling trace_interrupt() will now cause the read_packet functions
for these formats to exit, even if there are no packets available to
read.

Previously, the read_packet call would hang until a packet arrived
and then exit, which essentially meant tools like tracesplit would
hang forever if there were no packets.
Merge branch 'develop' of https://github.com/wanduow/libtrace into develop