Add middleware for catching large posts

This will return an error response that isn't currently picked up by
the JS Bin front end, but it does stop MySQL throwing exceptions. It
can be turned off by setting the "bin max-size" to 0 or null.
Add "bin max-size" option

This is set in bytes and limits the maximum size of a bin panel. The
default is currently 1.5Mb.
Add a 413 Request Entity Too Large error object
Include the limitContentLength() middleware in the app
Improve API for setting max request limit

Renamed the key to "max-request-size" and allow units to be provided
rather than just a number. The default is now 1MB.
Add very basic client error handling for large bins

This basically hijacks the tips notification and displays a message
informing the user the bin is too big.
Only parse the upload limit option once
Update YUI to the lastest version (3.8.0).
Fix the console replace regex. #432
Theoretical fix for internation keyboards with alt key.
Check for alt settings in both key handlers & remove global var.
Modify shortcut help to relect changes & add behaviour for useAlt toggle.
Copy & styling tweak.
Merge pull request #433 from remy/fix/console-text-432

Fix the console replace regex. #432
Merge pull request #434 from remy/fix/keys-392

Keyboard shortcuts
Merge pull request #430 from ericf/yui-3.8.0

Update YUI to the lastest version (3.8.0).
get gist support back in #382 - but it is auto-saving which I'm not keen on
version bump
Update to make mergable.
Merge branch 'master' into upload-limit