[WIP] Correlation time estimators

This PR now contains 5 estimators for the integrated autocorrelation time / statistical inefficiency. They are basically the ones from http://arxiv.org/pdf/1011.0175.pdf

- batch means
- linear fit to the frequency portion of the periodogram
- AR(p) model
- geyer's initial positive sequence
- `pymbar`'s esimator (truncating the window when the empirical acf crosses zero -- this is very similar to geyer's).

Still needs:
- [x] proper API / docstrings
- [x] support for multidimensional input data (many 1D AR(p), not VAR).
- [x] gets R installed on travis for py2 for tests
- [x] get R installed on travis for py3 for tests
- [x] sphinx docs

cc: https://github.com/choderalab/pymbar/issues/131