Add support for spell checking 

This PR adds an `webFrame.setSpellCheckProvider` API to set a provider for spell checking in input fields and text areas.

Example:

```javascript
require('web-frame').setSpellCheckProvider("en-US", true, {
  spellCheck: function(text) {
    return !(require('spellchecker').isMisspelled(text));
  }
});
```
![screen shot 2014-12-19 at 10 33 14 pm](https://cloud.githubusercontent.com/assets/639601/5513983/0ecebc60-87cf-11e4-86c8-c480fd5b8f86.png)


Fixes #724.