Add a polyfill for Function.prototype.bind

[rzane]

Function.prototype.bind isn't supported by all browsers. This PR just adds the polyfill mentioned here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind.

Alternatively, you could just not use bind and go with traditional this/that.

I personally use Poltergeist (which doesn't support Function.prototype.bind) as my javascript driver for Capybara and every test fails like this:

Capybara::Poltergeist::JavascriptError:
       One or more errors were raised in the Javascript code on the page. If you don't care about these errors, you can ignore them by setting js_errors: false in your Poltergeist configuration (see documentation for details).

       TypeError: 'undefined' is not a function (evaluating 'this.modal.find.bind(this.modal)')
       TypeError: 'undefined' is not a function (evaluating 'this.modal.find.bind(this.modal)')
           at http://127.0.0.1:37970/assets/application.js:23283
           at http://127.0.0.1:37970/assets/application.js:23693
           at http://127.0.0.1:37970/assets/application.js:375
           at http://127.0.0.1:37970/assets/application.js:140
           at http://127.0.0.1:37970/assets/application.js:23705
           at http://127.0.0.1:37970/assets/application.js:23710
           at http://127.0.0.1:37970/assets/application.js:3095
           at http://127.0.0.1:37970/assets/application.js:3207
           at http://127.0.0.1:37970/assets/application.js:3413
           at http://127.0.0.1:37970/assets/application.js:3429 in completed

[coveralls]

Coverage remained the same at 94.82% when pulling 407197d on rzane:polyfill-for-bind into 5b3bbc8 on volmer:master.

[volmer]

This is a good improvement. Thank you again for the contribution!