Compress by default, expose GC/CC option

Addresses code review comments. This now preserves the current default of
compressing DMD and GC/CC log files. The command-line flags have been
updated to specify if you want the files uncompressed.
Switch from xz to gz

This converts get_gc_cc_log.py to use gzip compression instead of xz which
brings it in line with the DMD logs. Additionally this removes the need to
shell out to an external process. |multiprocess.Pool| is used to preserve the
parallel compression behavior.