MAINT: Refactoring of chl.py

Refactored chl.py file to more closely match other Py-ART read functions.
ENH: Additional read_chl functionality.

* The `read_chl` function returns a Radar object with similar structure
  to other Py-ART read functions.
* CHL file type can now be detected and read by `pyart.io.read`.
* Added read_chl and chl.py file to documentation.
MAINT: Metadata for CHL files moved to config file.
MAINT: Default config for CHL file follows standard field names.
TST: Unit tests for chl module.
TST: debugging Travis CI fail
BUG: Fixed UTC time issue with CHL files.
Merge pull request #1 from jjhelmus/pr_160_enh

ENH: CHL file reader incorporated into Py-ART.