Localization, customization, a few bug fixes

Hi,

I'm making this pull request with all the things we've changed so far in the plugin. I'm not sure if you will want to integrate them all, or any, but it was easier to make a single pull request with everything so you can take a look at it first. Let me know if you want just a few things and I can make individual pull requests for them.

The biggest changes are:

* Enable localization: removed a few strings that were hard-coded, added language files for "en" and "pt_BR", changed the shortcode `[bigbluebutton]` to receive a few more parameters to customize strings and parts of the html it renders.
* Make it easier to customize the layout: added more classes and options so that the application can override the default css style.

This is the example of a `[bigbluebutton]` we use right now, including a few new options we needed to customize the strings and behavior:

```
[bigbluebutton token=123409s8uac submit="Entrar" name_label="Nome completo:" password_label="Senha:" waiting_text="Olá <strong>%USER%</strong>. A sessão ainda não começou, aguarde e você será redirecionado automaticamente assim que ela iniciar!<div class='loading'><div class='rect1'></div><div class='rect2'></div><div class='rect3'></div><div class='rect4'></div><div class='rect5'></div></div>" welcome="Olá, bem-vindo(a) à sala <b>%%CONFNAME%%</b>!"]
```

Also fixed a few small issues. You can see the complete list in the changelog (and in the commit messages too).

This is being used with WordPress 3.9.1.