Publish modifications to NPM on pull requests merge

Hi there,

It's been a long time since `wd` was last updated on NPM. The last published version is from the 3d of May (edb76fe3314fadb0f28420bcb8b514fcdcfdcb55), more than 80 commits have been made… and some features are really lacking, while they _have been developed_.

This is for example blocking our adoption of SauceLabs due to [lack of support for `uploadFile`](http://support.saucelabs.com/requests/5886), which was added in #122, 2 months ago.

It is therefore rather urgent to publish the current version to NPM, but it would be important to take the habit of publishing updates on (series of) pull request merges, to prevent lack of sync to get out of hand.

The current changeset is an incentive to publish: simply merge, pull and `npm publish`, you don't even need to manually bump the version number  ;)

Also, as advised by @sebv: ping @admc @jlipps @bernii  :)

Regards,
Matti Schneider