Ran pyflakes to remove extra imports

So I ran pyflakes to clean up some redundant imports etc.  There are still a few things that come up, which it might be worth looking into:

```
kyleb@kyleb-VirtualBox:~/src/choderalab/pymbar$ pyflakes pymbar/*.py
pymbar/mbar.py:846: local variable 'A_i' is assigned to but never used
pymbar/mbar.py:1483: local variable 'N_k' is assigned to but never used
pymbar/mbar.py:1900: local variable 'K' is assigned to but never used
pymbar/mbar.py:1901: local variable 'f_k' is assigned to but never used
pymbar/mbar.py:1902: local variable 'N_k' is assigned to but never used
pymbar/mbar.py:1903: local variable 'u_kln' is assigned to but never used
pymbar/mbar.py:1934: local variable 'info' is assigned to but never used
pymbar/timeseries.py:434: local variable 'g' is assigned to but never used
pymbar/timeseries.py:634: list comprehension redefines 'A_t' from line 620
```