Adding LoadSequentialAtlases

Automatically loads atlases with a certain numeric pattern, ex :
myAtlasName_0, myAtlasName_1 (this is how texture packer names/numbers
stuff)
Load atlases without using resource suffix

See this pull request: https://github.com/MattRix/Futile/pull/218

This allows people to mix atlases that have resource suffixes with those
that don't.