Adds Tile class and along some minor improvements.
Adds rules to docs how to use custom util.js.
Starts to implement behavior.

- adds util.prototype ignore flag on attribute creation
Adds config based behavior update time.
Default implementation for behavior and dirty on game#update.
Refactors unnecessary conditional statement.
Fixes using undefined variable for tick count reset.
Uses conditional assign instead of logical eval.
Merge branch 'feature/behavior' into develop
Updates TODO.md
Updates versions and docs.
Merge branch 'release/0.4.0'