Makes my stupid test script check if the return file is the same
Stuff for #5

- CloudConvert.upload() was making a sightly bad request.
- ConversionProcess.is_possible() moved to CloudConvert. It was useless
since the API already complains when the conversion is impossible.
- Refactored a little bit how ConversionProcess.init() detects the file
extension, so my linter doesn't complain everytime about the line being
too long.
- The call to the API made by ConversionProcess.start() sent the origin
format, when the parameter was suppposed to be the output format. And
it's been like this since the start :(