use HTML5 download attribute rather than hitting a server

[dsjoerg]

For those who would rather not hit a server to simply export a CSV, you can use the code in this PR.

Browser compatibility here: http://caniuse.com/download

As of March 2014, works on current Chrome, Firefox, Opera, and others.

However, due to lack of compatibility on IE and Safari, I don't expect this PR can be accepted anytime soon.

[TorsteinHonsi]

However, due to lack of compatibility on IE and Safari, I don't expect this PR can be accepted anytime soon.

If you are able to throw in some feature detection we could pull it.

[raveren]

Here is the detection code:

var downloadAttrSupported = ("download" in document.createElement("a"));

taken from http://stackoverflow.com/questions/12112844/how-to-detect-support-for-the-html5-download-attribute#comment16193018_12112905