cleanup instance checks in pandas/tests, add assert_isinstance method

and also pandas/tseries/tests, pandas/stats/tests
CLN: stats/tests, sparse/tests - use relative imports
relative import in rpy
CLN: relative imports, instance checks in pandas/io
CLN: unicode --> six.text_type
CLN: Python 2/3 compatible sort in ez_setup
change 1 to True in doc
TST/ENH: Add assert_isinstance util to util/testing
CLN: Modernize pandas/tests + better type checks

Use assert_isinstance + replace type(a) == b with assert_isinstance.
Some 2/3 fixes of .sort() --> sorted
Better instance tests in io/tests
CLN: assert_(isinstance --> assert_isinstace
CLN: 2/3 fixes to util/decorators
CLN: better instance checks in scripts/
CLN: better type check in perf_HEAD
CLN: Use compatible long in io/stata
CLN: Cleanup type checks in core/config

Make is_instance_factory cleaner + basestring --> (str, bytes)
fixup for wrong isinstance change