PEIClient adds support for latest get_load(...) and get_generation(...) values

[r24mille]

The Prince Edward Island implementation uses data from this page: http://www.gov.pe.ca/windenergy/chart.php Only latest=True options are supported for PEIClient implementations of get_load(...) and get_generation(...) The derivation for this data is roughly as follows:

• get_load(...) is the "Total On-Island Load" value.
• get_generation(...) fuels are determined as follows:
  • wind is the "Total On-Island Wind Generated" value.
  • oil is the "Total On-Island Fossil Fueled Generation" value. The description for this fuel is, "the amount of electricity being generated from oil fired equipment. Typically, this generation is only required when there is an interruption of supply from off Island."
  • other is "Total On-Island Load" - wind - oil. In my understanding this is largely imported.

I originally felt that deriving get_generation(...) fuel mix from these charts was dubious (see #129. Looking at the PEI's data the "Wind Power Used On-Island" + "Wind Power Exported Off-Island" = "Total On-Island Wind Generated". The percentage value they give for "Total On-Island Wind Generated % of full load" is the percentage of "Total On-Island Load". So if the pyiso definition of "generation mix" is the mixture of fuels required used to supply the total load (i.e. internal load + exports + transmission losses) then I think my derivation of fuels for get_generation(...) is fine.

Closes #129.

[coveralls]

Coverage increased (+1.04%) to 38.454% when pulling dead5542c3195069c57cfae1ee0f8980ffdf2fdb on r24mille:129_pei_client into 1107696c11a90c8554ccc4b810ae7dff9e20e618 on WattTime:master.