Add Cheetah and Jinja2 comparisons to render benchmark

[nicksay]

Example output:

$ python tests/benchmarks/render_benchmark.py  --compare --number 1000
Running benchmarks 1000 times each...

Cheetah template                               18.28 ms
Jinja2 template                                 8.84 ms
Jinja2 template autoescaped                    18.95 ms
Python string template                         30.92 ms
Python StringIO buffer                         20.20 ms
Python cStringIO buffer                         6.06 ms
Python list concatenation                       2.29 ms
Spitfire template                              11.05 ms
Spitfire template -O1                           9.81 ms
Spitfire template -O2                           6.84 ms
Spitfire template -O3                           6.63 ms
Spitfire template baked                        13.00 ms
Spitfire template baked -O1                     8.34 ms
Spitfire template baked -O2                     8.37 ms
Spitfire template baked -O3                     8.25 ms
Spitfire template unfiltered                    6.24 ms
Spitfire template unfiltered -O1                2.76 ms
Spitfire template unfiltered -O2                2.76 ms
Spitfire template unfiltered -O3                2.18 ms

[awbraunstein]

lgtm