BUG, ENH: Improve infinity parsing for read_csv
1) Allow mixed-case infinity strings for the Python engine

Bug was traced back via `lib.maybe_convert_numeric` to the `floatify` function in `pandas/src/parse_helper.h`.  In addition to correcting the bug and adding tests for it, this PR also moves the `test_inf_parsing` test from `c_parser_only.py` to `common.py` in the `pandas/io/tests/parser` dir.

2) Interpret `+inf` as positive infinity for both engines

`float('+inf')` in Python is interpreted as positive infinity, so we should allow it too in parsing.
