Fix Enter erroring due to broken 'enter' handler #ImAnIdiot
Factor out handlers to Controller #simplify

Using better, more reliable strategy to determine whether to pass along
`dir` or not, which deals with smell of checking the index of the loop:
https://github.com/mathquill/mathquill/pull/351#issuecomment-62074898
Merge pull request #353 from mathquill/fix.enter-handler

Fix Enter erroring due to broken 'enter' handler #ImAnIdiot
Revert "Update fork from dev"

This reverts commit 6a200efa490a73f1b913d2d5f531c2fecda0c7d2.
Merge branch 'dev' of https://github.com/mathquill/mathquill into dev