Added mode for GNU assembler syntax.

The mode parses the GNU assembler syntax (also known as gas or AT&T
syntax.  An "architecture" option can be provided to the mode which
will cause the parser to accept machine specific syntax.  Currently
the only options accepted are "ARM", "ARMv6" and "x86".
[gas mode] Fix overly precise comparisons
[gas mode] Untabify
[gas mode] Integrate
Prevent crashing on IE when in iframes

"Unspecified error" on document.activeElement when
running in an iframe in IE.
Make text inside hidden textarea tiny

To prevent it from pushing the scroll position of the wrapper.
More careful checking of viewport coverage after updateDisplayInner

Closes #1391
Fix multiplex close on blank line.
[gas mode] Fix undefined var error
Prevent bogus error messages when a mode throws an error
Add maxHighlightLength option
Reset hidden textarea when swapping an editor's document
Merge branch 'master' of https://github.com/marijnh/CodeMirror