Adds BnetScraper::Starcraft2::Portraite class

Instead of only returning the portrait name, a Portrait class has been added, and the
logic for translating the scraped data into coherent information has been extracted
from ProfileScraper into Portrait.  This includes methods to access the spritesheet
as well as singular portrait image
Documentation improvements for Portrait
Portrait now takes 4 params instead of an array

BnetScraper::Starcraft2::Portrait.new takes 4 Fixnums instead of an array of 4 Strings. The
ProfileScraper handles typecasting the strings into Fixnums to cut down on the to_i.