#33 Move to TreeCorr for correlation function tests

This is a PR to merge code that uses the TreeCorr package rather than corr2.  A lot of this is inside baseball for the workings of the CorrelationFunctionSysTest, but I will highlight in particular:
 - Most of the code is now gone from `corr2_utils.py` (now `treecorr_utils.py`).  You have no idea how excited I am about this.
 - The call signature for the CorrelationFunctionSysTests has changed: now it is
```sys_test(data, [data2, random, random2,] config=config_dict, **kwargs)```
instead of the old
```sys_test(config_dict, data, [data2, random, random2], **kwargs).```
The new form is more in line with the call signatures of the other SysTests, and I think it's clearer too.
 - Various places that "corr2" has been changed to "TreeCorr."
 - As long as I was changing imports, I went through and tidied up extraneous import calls in all files in the `stile` directory (all of which were, I'm sure, my fault in the first place).
 - Added a `fieldNames` dict to `stile_utils.py`, so we have an obvious in-Python place to look up what the fields should be called.  (Possibly we should do this for `object_type`s too?)
 - General PEP8ifying (spaces after commas, no trailing spaces on lines, etc)
 - Added a CHANGELOG.

Unfortunately, due to both rewriting a lot of the file & changing its name, the GitHub interface thinks `stile/treecorr_utils.py` and `stile/test_treecorr_utils.py` are entirely new files.  Hopefully it's not too terrible to read, though.
Philosophical question:
Are we essentially allowing the user to specify any of the TreeCorr options?
I'm just thinking that this piece of code is going to have to be kept in sync with any TreeCorr changes - if there's a new release that changes one of these, then we have to modify our code.  Obviously some of them are necessary but if there are any that are unlikely to be needed for standard Stile usage, we might not want to include it as something that user can change.  That way we'll be less susceptible to issues due to changes in the TreeCorr API.