Merge pull request #1408 from QuLogic/pyugrid-optional

Make pyugrid truly optional
Remove iter* from dicts.
Make cartopy optional (at build time, at least).
Use configparser instead of ConfigParser.
Use zip_longest instead of izip_longest.
Use MutableMapping instead of DictMixin.
Encode string passed to C function.
Temporarily switch out exec lines.
Skip _init call. Maybe should be super()?
Enable Python 3 on Travis.
Install different Conda requirements on Python 3.

They have yet to be actually determined, though.
Pandas needs an update on Py3.
py3: Cartopy is not available yet.
Install many things via pip instead of conda.
Remove pyugrid on Py3.
Py3: Don't preload Cartopy data yet.
Revert to building grib from source for now.
Fix install of Python module from grib.
Restore travis printing.
Missing semicolons.