Add a python tool to produce a diff of two logcat files

This can be used to narrow down differences between two runs of Android
and help triage issues

The tool strips most information from logs, reducing it to the component name
and the message string, it then processes the output of this through
diff (and colordiff for ease of reading)

Bug: None
Test: logcat_diff.py log1 log2
      and observe desired output
Change-Id: I70ac765e4ed3f52a84fe136c080f5cf543737748