Fix for ST3 (and Linux Mint)

Trying to setup this plugin for my new ST3 installation I run into a couple of exceptions. After tinkering with it for a bit, I found these solutions.

The first exception was complaining about line 138 in ExportHtmlLib/desktop/__init__.py with the `endswith()` apparently not getting bytes as the first argument, but a string, which I resolved by applying `encode()` to that argument.

The second exception is most likely Linux Mint specific, and was complaining about that "neither DESKTOP_LAUNCH nor os.startfile could be used". I found a solution suggested [here](https://github.com/revolunet/sublimetext-markdown-preview/issues/21#issuecomment-16376254) which solved the problem and finally got the plugin to work properly.