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 to make mergable.
Merge branch 'master' into upload-limit
Merge pull request #377 from remy/upload-limit

Check bin content file size before parsing request