Leukeleu-drf-tus

* Fixed bug that occured when pausing uploads (https://github.com/dirkmoors/drf-tus/pull/7)

* Make the metadata 'filename' key a setting. (https://github.com/dirkmoors/drf-tus/pull/2)
* Switch from (Positive)IntegerField to BigIntegerField for the upload_length and upload_offset fields (https://github.com/dirkmoors/drf-tus/pull/3)
* Increase Django version support up to 1.11 (https://github.com/dirkmoors/drf-tus/pull/4)
* Fix unicode errors (https://github.com/dirkmoors/drf-tus/pull/5)
* Updated travis configuration

* Exchanged use of `base64.encodebytes` to `base64.b64encode`.

* Implemented `validate_filename` handler in `TusCreateMixin`.

* Added missing migration and changed `max_length` to 255 for model `Upload`'s attribute `uploaded_file`

* Updated default behaviour of the `get_header` handler in `TusMiddleware`.