Flixel v2.56

This version fixes many of Flixel's older bugs, while staying reverse compatible
with projects that use Flixel v2.55 (the current version in
[AdamAtomic/flixel](https://github.com/AdamAtomic/flixel)).

For complete changes, see CHANGELOG.md
Increment version number to v2.57

As we are using odd numbers for dev versions and even numbers for
release versions, this will eventually merge into master as v2.58.