Add Kane benchmark

I include the generated long expression. I haven't decided whether I should include it in git, or just let it regenerate using the included `kane_generate.py` script, but I am leaning towards the latter.

Results on my computer as of 31378da:
```
ondrej@hawk:~/repos/csympy/benchmarks(kane)$ python kane.py 
Converting to SymPy...
Converting to CSymPy...
    Done.
SymPy subs:
Total time: 0.059886932373 s
CSymPy subs:
Total time: 0.112519025803 s
SymPy diff:
Total time: 5.41207313538 s
CSymPy diff:
Total time: 0.194560050964 s
```

As of bbd4109 (I did some speedup of the diff method):
```
$ python kane.py 
Converting to SymPy...
Converting to CSymPy...
    Done.
SymPy subs:
Total time: 0.0590569972992 s
CSymPy subs:
Total time: 0.109745979309 s
SymPy diff:
Total time: 5.41652584076 s
CSymPy diff:
Total time: 0.166007041931 s
```
As of fb7ebee (some speedup of subs):
```
$ python kane.py 
Converting to SymPy...
Converting to CSymPy...
    Done.
SymPy subs:
Total time: 0.0594460964203 s
CSymPy subs:
Total time: 0.0626931190491 s
SymPy diff:
Total time: 5.39876413345 s
CSymPy diff:
Total time: 0.169718980789 s
```
As of e9525c7 (more subs speedup):
```
$ python kane.py 
Converting to SymPy...
Converting to CSymPy...
    Done.
SymPy subs:
Total time: 0.0599641799927 s
CSymPy subs:
Total time: 0.0462651252747 s
SymPy diff:
Total time: 5.41399884224 s
CSymPy diff:
Total time: 0.171603918076 s
```
SymPy's sub is almost as fast as CSymPy. I wonder whether there is any way to speedup CSymPy further.
The diff is now almost 32x faster. Not bad, but I hope there is still some possibility to speed things up further.

@gilbertgede, @hazelnusse, is this benchmark meaningful at all? Am I benchmarking the right thing?