Add error callback to webkitGetUserMedia, removed grunt-cli as npm dependency.

`webkitGetUserMedia` expects 3 parameters (see https://github.com/thieman/github-selfies/pull/24).

Removed `grunt-cli` from package.json as it is supposed to be installed globally (could be enforced by adding
```
  "scripts": {
    "postinstall": "npm install -g grunt-cli"
  }
```
to `package.json` instead).