Pandas assignments

Here is driver (and testing) code:

```
import pandas as pd
from msmbuilder import assignments, io, MSMLib

ass_msmb2 = io.loadh("./Ward-3120/Assignments.Fixed.h5")["Data"]
ass = assignments.convert_array_to_pandas(ass_msmb2)

C = assignments.get_count_matrix_from_assignments(ass)
C0 = MSMLib.get_count_matrix_from_assignments(ass_msmb2)

In [9]: C - C0
Out[9]: 
<3115x3115 sparse matrix of type '<type 'numpy.float64'>'
	with 0 stored elements in Compressed Sparse Row format>
```