run: memoize the association between trace_names and unique words to speed up parsing
run: memoize the association between trace_names and unique words to speed up parsing

By avoiding the getattr and all the references to the Run classes I
get a speed up of 50% on my laptop on a 27Mb trace file:

Before:

In [1]: %timeit cr2.Run("/home/javi/src/thermal/results/2014-11-20/odroid-power_allocator-47f694f451b4-2/transporter_5_1/")
1 loops, best of 3: 4.66 s per loop

After:

In [1]: %timeit cr2.Run("/home/javi/src/thermal/results/2014-11-20/odroid-power_allocator-47f694f451b4-2/transporter_5_1/")
1 loops, best of 3: 3.09 s per loop

Change-Id: Id7b8132e5e26ffb1f55e2a09ed1f250720685647

bjackman/trappy,sinkap/trappy,sinkap/trappy,JaviMerino/trappy,joelagnel/trappy,ARM-software/trappy,JaviMerino/trappy,derkling/trappy,sinkap/trappy,ARM-software/trappy,joelagnel/trappy,joelagnel/trappy,derkling/trappy,ARM-software/trappy,joelagnel/trappy,bjackman/trappy,ARM-software/trappy,sinkap/trappy,bjackman/trappy,bjackman/trappy,derkling/trappy,JaviMerino/trappy