TextField auto-sizing

(Re-created to merge cleanly with the latest Starling HEAD)

This adds auto-sizing capabilities to TextField, and is intended to address the #169 open issue.

Adds the ability to have TextFields automatically adjust their bounds
to the bounds of their text contents.

This works differently from Flash's native TextFieldAutoSize feature,
which is notoriously hard to use, and confusingly conflates auto-
sizing and alignment. There are two types of auto-sizing: SINGLE_LINE
and MULTILINE, which behave as expected. Use
TextField.autoSizeMaxWidth and autoSizeMaxHeight to control how auto-
sized text will be laid out.

Auto-sizing is incompatible with Starling's existing
TextField.autoScale feature; enabling one will disable the other.