Check if errorList is not null
More efficient chunk merging.

We must not read the file contents into memory, especially if we deal
with large amount of data which is the main use case of chunked uploads.

This commit fixes the old behavior and introduces a more efficient merging.
We still read every chunk into memory but this happens in a generator
and we do not merge in-memory so that the memory usage stays constant.
Use resumableCurrentChunkSize parameter
Merge branch 'master' of github.com:jeanphix/django-resumable into efficient_chunk_merging
move pattern creation out of for loop
Merge pull request #3 from EnTeQuAk/efficient_chunk_merging

More efficient chunk merging