Spellchecker languages

Hi,

In our recent upgrade from TinyMCE 3, we found that users were relying on the support for multiple languages in the spellchecker plugin. Support for this seems to have been dropped in TinyMCE 4.

I've spent a bit of time adding this back in. The plugin checks the `spellchecker_languages` setting, using the default list of languages from TinyMCE 3 if it's not specified. If it's specified, it's in the familiar `Language=code,Language=code` format as before, although without the `+` for specifying the default as it determines this from TinyMCE's own `language` setting.

I've added a skeletal unit test file with examples of the initialisation and to test the button/splitbutton behaviour. This doesn't test the actual spellcheck behaviour, but that's unchanged anyway.

It would be nice to see this in master if it's useful to anyone else! :)