feature/define-check-base

[cosmicBboy]

This PR makes a couple of changes:

• define a _CheckBase class the children of which are Check and Hypothesis.
  This cleans up the Hypothesis API, as the Check built-ins were accessible to
  Hypothesis before this change
• clean up a bunch of things in the docs, mainly add the __call__ special member to
  the API reference and add class attributes.

[codecov-io]

Codecov Report

Merging #195 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   96.21%   96.22%           
=======================================
  Files           9        9           
  Lines         714      715    +1     
=======================================
+ Hits          687      688    +1     
  Misses         27       27           

Impacted Files	Coverage Δ
pandera/checks.py	98.42% <100.00%> (+<0.01%)	⬆️
pandera/error_formatters.py	100.00% <100.00%> (ø)
pandera/hypotheses.py	86.79% <100.00%> (ø)
pandera/schema_components.py	98.21% <100.00%> (-0.02%)	⬇️
pandera/schemas.py	96.74% <100.00%> (ø)

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