Correct datetime test issues due to latest pandas versions

[serazing]

• Update minimum requirements to pandas >= 0.22
• Add a function to test if an object has a datetime dtype because the error referenced here #10 have been duplicated in other pieces of the code. This should increase the code robustness.

[coveralls]

Coverage increased (+0.08%) to 53.748% when pulling b978a55cfe945c2ed9820a61b8754edb356bb2ef on fix_pandas_issue into 4c6a0dc501256d8ac2f4c792a79aa5efad29a41a on master.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.08%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   53.14%   53.23%   +0.08%     
==========================================
  Files          11       11              
  Lines        1080     1082       +2     
==========================================
+ Hits          574      576       +2     
  Misses        506      506

Impacted Files	Coverage Δ
xscale/_utils.py	92.98% <100%> (+0.12%)	:arrow_up:
xscale/signal/fitting.py	86.55% <100%> (ø)	:arrow_up:

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