EIA client

[andydevlinsmith]

This addresses https://github.com/WattTime/pyiso/issues/97.

This is my first PR for this project, so constructive feedback would be welcome.

Caveats:

• We may need to add an EIA key to the CI tool for the tests to pass. I can give you mine or we can set one up for Watttime and use that.
• I hit some issues pulling Canada/Mexico data, so this just includes US BAs for now.
• Some BAs don’t consistently return some data, as documented in the problem_bas lists in the tests. This probably needs more graceful handling.
• The setup for tests was different enough that I found it easiest to put everything up in test_eia rather than incorporate them into test_load/test_genmix/test_trade.
• Generally speaking, I'm sure this can improve but figured I would get some input before going further.

Thanks!

[andydevlinsmith]

Many thanks for the comments! I'll get these in and update the PR.