Template -> Processor

Closes #681
Add shorthand for charset detecter
Keep at least 2 backups and any created within an hour
Disable integrity manifest writing
Tweak var names
Merge pull request #684 from sstephenson/keep-recent-backups

Keep recent backups within a time window
Remove unused var
Allow new instances of DirectiveProcessor
Use correct comment style
Allow directive processor comment style to be configured

Closes #674
Cache singleton instances
Merge branch 'master' into processor-cache-key

Conflicts:
	lib/sprockets/closure_compressor.rb
	lib/sprockets/coffee_script_template.rb
	lib/sprockets/eco_template.rb
	lib/sprockets/ejs_template.rb
	lib/sprockets/erb_template.rb
	lib/sprockets/sass_compressor.rb
	lib/sprockets/sass_template.rb
	lib/sprockets/uglifier_compressor.rb
	lib/sprockets/yui_compressor.rb