CLN: clean up parser options

Also add a test to make sure that the C parser options validation is actually
covered
CLN: remove factorize argument
Merge pull request #5121 from cpcloud/parser-options-mania

CLN: clean up parser options
BUG: set_names should not change is_ relationship
Merge pull request #5132 from jtratner/fix-set-names-error

BUG: set_names should not change is_ relationship
BUG: Treat a list/ndarray identically for iloc indexing with list-like (GH5006)
Merge pull request #5134 from jreback/list_index

BUG: Treat a list/ndarray identically for iloc indexing with list-like (GH5006)
ENH: Json support for datetime.time
ENH: json default_handler param
Merge pull request #5137 from Komnomnomnom/json-time-suport

ENH: Json support for datetime.time
Merge branch 'json-default-handler' of https://github.com/Komnomnomnom/pandas into Komnomnomnom-json-default-handler
TST: fillna(values) equiv of replace(np.nan,values)
Merge pull request #5139 from jreback/fillna_string

TST: fillna(values) equiv of replace(np.nan,values)
BUG: MultiIndex.get_level_values() replaces NA by another value (#5074)
Make _na_value a class attribute.