Feature/encodings

This PR implements idea discussed in #138.

It does two things:
  - remove chardet and autodection
  - add support for choosing encoding (EncodingPanel, EncodingComboBox, PreferredEncodingsEditor).

This PR is not finished yet, I have still to add a few more widgets (EncodingErrorDialog, EncodingMenu) and add a proper test suite to cover the new API.

I will be on holidays the next days so do not expect this PR to merged before the middle of the end of the next week.


Here is what it looks at the moment:

1) EncodingPanel in case of decoding error:

![1](https://cloud.githubusercontent.com/assets/1681217/3518347/07c38802-06ff-11e4-9f6a-0e9ba080bf9f.png)

2) You can choose a new encoding to retry from the combo box. To keep it small, the menu of the combobox only contains your preferred encodings. Clicking on add/remove brings the following editor:

![2](https://cloud.githubusercontent.com/assets/1681217/3518349/07c5c734-06ff-11e4-9c96-7c442c2970a3.png)

3) Now you can retry with the encodings you just added

![3](https://cloud.githubusercontent.com/assets/1681217/3518348/07c41d8a-06ff-11e4-943b-2b3da5fb955a.png)

4) Yeah that works!

![4](https://cloud.githubusercontent.com/assets/1681217/3518350/07c6eb0a-06ff-11e4-9e7f-c07900aad271.png)