Make import styles in init consistent.

 re #49 

My point here is to avoid encouraging users to do `import pymbar;pymbar.pymbar.something`.  The new namespace looks like this:

```
In [1]: import pymbar

In [2]: pymbar.
pymbar.bar                  pymbar.BARzero              pymbar.exp                  pymbar.EXPGauss             pymbar.MBAR                 pymbar.timeseries           pymbar.version
pymbar.BAR                  pymbar.confidenceintervals  pymbar.EXP                  pymbar.mbar                 pymbar.testsystems          pymbar.utils                

```