Integrate parsing of definition files into main holidays logic

Moves the parsing of holiday definition files into the main
holidays codebase. The core logic is the same but it has been split
into different methods for clarity. Some variables have also been
slightly renamed for clarity.

No behavior change with this commit.
Add ability to load custom holiday definitions on the fly

Allows for the loading of custom holiday definition files on the
fly. Parsed definition files are immediately loaded and can be
referenced like any existing definitions.
Merge remote-tracking branch 'origin/master' into load_custom_holidays

Conflicts:
	README.rdoc
	rakefile.rb
Load Custom Holidays: fix Rakefile after merge
Merge pull request #57 from ptrimble/load_custom_holidays

Add ability to load custom holidays on the fly