[feature] allow to choose jpg format

Hi !

First of all : Thanks a lot for your plugin !!

PNG is great for big uniform areas (vector-style maps), but poor for more complex images (satellite-style maps). For such images. with reasonable quality (around 80), tile size drops from around 160kb to 25kb.

So I implemented possibility to choose JPG format to export.

![tojpg](https://cloud.githubusercontent.com/assets/1894106/3256338/2817b422-f217-11e3-9f72-16252948f754.png)

This PR replaces the "suffix" (not sure what this was good for?) line edit by a "format" combobox. The suffix is then automatically set by the format.
It adds a "quality" spinbox (0-100), which is enabled only for the jpg format. The transparency spinbox is disabled when jpg format is chosen.


/!\ IMPORTANT BEFORE MERGE /!\
I don't know the MBTile format at all, and didn't test it.
I'm not sure if it depends on the png format or not. If so, this PR breaks it, since it also stores the data as JPG in the sqlite table.
I found some references to PNG in mbutils.py, but the parts using it seemed unused.