BLD: New R versions require NAMESPACE file. Closes #1497.
Merge pull request #1504 from jseabold/namespace-for-r-package

BLD: New R versions require NAMESPACE file. Closes #1497.
TST: Test co2 load_data for Python 3.
BUG: Decode dates to strings.
Merge pull request #1505 from jseabold/to-datetime-py3

COMPAT: Make sure co2 load_data works for Python 3.
BUG: Format and decode errors in Python 2.6

Fixes two small errors on Python 2.6
- Improper use of keyword arguments in format, e.g errors='strict'
- Improper use of position-less format string
Merge pull request #1506 from bashtage/python-2-6-fixes

COMPAT: Fix format and decode errors in Python 2.6
DOC: Fix formula import convention.
DOC: Fix broken link
Merge pull request #1514 from jseabold/fix-formula-import

DOC: Fix formula import convention.
Merge pull request #1515 from jseabold/fix-docs-link

DOC: Fix broken link
DOC: Be clear about data attribute of Datasets
Merge pull request #1517 from jseabold/datasets-docs

DOC: Be clear about data attribute of Datasets
DOC: Update examples contributing docs for current practice.
Merge pull request #1516 from jseabold/uploading-examples

DOC: Update examples contributing docs for current practice.
DOC: Ask for release notes and example.
Merge pull request #1518 from jseabold/contributing-docs

DOC: Ask for release notes and example.
ENH: Unit Root Tests

Introduces 4 new unit root tests and provides a class for Augmented Dickey Fuller testing.
The four new tests are:
  - PhillipsPerron
  - KPSS
  - Variance Ratio
  - DF-GLS

These have all been included in unitroot.py, and the afduller function has been moved from
tsatools to unitroot, and is now just a wrapper around the ADF class.

The critical value and p-value lookup codes have also been consolidated into untiroot.py.
Critical value and p-value lookup information for DFGLS and KPSS is not readily available
in published form, and so simulations were performed to generate these using 100,000,000
replications using a methodology similar to MacKinnon (2010).  All critical value data
has been consolidated into tsa/critical_values.

A cov_nw function was added to tsa/stattools which computes Newey-West covariance.

Also includes substantial improvements to tsatools, including numerous bug fixes.   The
code that handled recarrays has been improved to now handle pandas dataframes.  Numerous
doc strings where improved.  Test coverage was also substantially increased.

string_types was added to the compatibility library to address the difference between basestring
in Python 2 and str in Python 3.

Moved coint to a new file coinegration.py which will, in the long term, be expanded to include
other tests for cointegration.