Add AESOClient implementation of BaseClient to provide data for Alberta, Canada

[r24mille]

The Alberta Electric System Operator (AESO) has legislative obligations to make their data public. You can get a sense for some of it at: http://ets.aeso.ca/

It's rough, but CSV data is available at: http://ets.aeso.ca/ets_web/ip/Market/Reports/CSDReportServlet?contentType=csv

We could easily add latest support for get_generation(...) using the fuels, get_trade(...) using Net Actual Interchange, and get_load(...) using Alberta Internal Load (AIL).

An AESOClient would provide value, since currently the EIAClient lists the AESO in its canada_mexico list, meaning that data is not supported.

[r24mille]

Upon looking into the AESO reports a bit closer, Historical start_at (back to January 2000) and a forecast end_at through the end-of-day could be added for get_load(...) using the report using Historical > TRADING > Actual Forecasts for historical data. You can request batches of 31 days. The following link provides forecasts through the end-of-day:

http://ets.aeso.ca/ets_web/ip/Market/Reports/ActualForecastWMRQHReportServlet?contentType=csv

Furthermore the Historical > TRADING > System Marginal Price could be requested for historical values (and latest) if it's analogous-enough to pyiso's get_lmp(...) intentions. It's basically a single-node LMP.

[r24mille]

Completed with #121.