Improve keyboard shortcut to submit comments

[rmasoni]

It should accept cmd+Enter on OS X, just like GitHub.

Also, it's apparent that just before submitting, it's creating a line break. See: http://quick.as/vd2wcrqe
@Gissues:{"order":62.5,"status":"notstarted"}

[sebastian-code]

We'll have to check this out to validate how it's working

[sebastian-code]

Closing this issue because is too old, adding it to #66 to keep track of it.