Improve mpg123 performance

Hi,

first of all: Thanks for creating bongo. I just started to use it and really like it!

I also noticed high CPU load:
https://github.com/dbrock/bongo/issues/3

and did some profiling/improvements:

High CPU load is produced by parsing mpg123 process output.  Instead
of creating a temporary buffer for each process output and using
buffer-search/regex functions to parse it, transform input string to
an intermediate list representation by using faster string functions.

Also remove duplicate events: By rounding the elapsed time to full
seconds a lot of events don't have to be processed.

This reduces CPU load from 60% to 24% on my Core2Duo system.