ENH: Add regex=True flag to str_contains
Using regex=False can be faster when full regex searching is not needed.
See http://stackoverflow.com/q/20951840/190597

TST: Add a test for str_contains with regex=False
BUG: Not all strings.str_* functions return an object with an ndim attribute.
PERF: add benchmarks for every str method
Merge pull request #5879 from unutbu/str-contains

ENH: Add regex=True flag to str_contains