REF: ensure O(N log N) when using fft for acf

The numpy implementation will fall back to a O(N**2) version of the fft
in cases where it cannot factorize the array length, which can be a
increase in runtime of several order of magnitude. This uses the
_next_regular function from scipy to ensure that we always use array
lengths for which fftpack is optimized.
TST: add test for _next_regular

also copy the test for _next_regular from scipy
ENH: Option to avoid "center" environment.
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.