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.
REF: Refactor arfilter. Correct name and split. Closes #1478.
ENH: Add seasonal_decompose function.
ENH: Use pandas nanmean. Numpy too new.
TST: Old pandas can't assign nan to int series.
REF: Convert to array before checking finiteness.
ENH: Let non-ts pandas objects through. Need to set freq.
ENH: Return pass through function
ENH: Check MS not MS- for freq
ENH: Pull out seasonal_mean function.
Made Exponential Smoothing for Statsmodel

Started Exponential Model off of code from dfrusdn and heavily modified. Handles 15 different models.
Update Expsmoothing

remove lines some style
Rename Expsmoothing to _expsmoothing

Changed title for cython move recommended by josef
Update _expsmoothing

added authors per josef
REF: Add file ending
STY: Pep-8 and docstring cleanup