Since v2 is the canonical CodeMirror at this point

Courtesy of Alexey Kolosov

Closes #283

Added getTextarea() method which returns the original textarea element when CodeMirror is instantiated using fromTextarea() method.

Context in pull req #281

This should prevent problems with parts of the document having a 'corrupt'
state when quickly scrolling down a big document.

Apparently this works around a Mobile Safari quirck.

It was causing strange event-handling artifacts on windows.

Issue #274

Closes #272

It was accidentally sharing a variable with the charWidth cache, and using its
measureText string before it was initialized.

Add: mark not matching closing bracket as error

Concepts: indention matters only for python statements. So I made it explicit: instead of creating more scopes for different indention, we just change offset of "enclosing" scope; that way inside brackets "previous line indention" strategy works for new lines.

This way, if you load the htmlmixed mode before the PHP mode, it won't
try to use that.