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.

ok looks reasonable. pls run parser asv's just to be sure (I doubt we have something that specifically has infs in it; you can add if you want).
 btw, not averse to signed inf (e.g. `+inf`), if its in the IEEE spec (not sure)
 1) Not sure if it's necessary to add a benchmark for this.  A couple of string checks shouldn't have much effect, so unless there is a strong objection, I'll leave it for now.

2) I can't actually run benchmarks at the moment.  There seems to be an issue with linux-64 channels in Anaconda for `tables` and `libpython` that prevents me from running the benchmarks.  Can you confirm on a different architecture?

3) Can't confirm if `+inf` is in IEEE standard (Googled "IEEE infinity," and that didn't turn up anything).  _However_, `float('+inf')` does work in Python, so I guess we should accept it.  However, it is not supported in the CParser either right now, so I'll need to see where infinity parsing is done there.
 pls just run the current benchmarks - don't need a new one

you don't need libpython on Linux
 I'm just running `asv continuous master floatify-infinity-parsing -b io_bench`, and that's what it's complaining about, nothing special here, right?
 @jreback : Added `+inf` parsing to both parsers, and Travis is still giving the green light.  While I cannot run the benchmarks because of that Anaconda issue, I wouldn't be surprised if they remain relatively the same since my changes shouldn't down anything too much.  Ready to merge if there are no other concerns.
 lgtm, ping on green. we don't have an associated issue right?
 Nope.  Discovered it when I was refactoring `test_parsers`, and someone had inadvertently put `read_csv` instead of `self.read_csv` for that test I moved in this PR.
 @jreback : Travis giving the green light .  Ready to merge if there is nothing else.
 nice PR @gfyoung 

only comment is that don't put Bug Fixes at the very end, causes conflicts, stick them somewhat randomly in the spaces  within the current list (that's why I leave the space)
 @jreback : Ah, fair point.  Bad habit on my part.  Thanks for the reminder!
