Addresses client-side solution for #694

Added requests-toolbelt as a new project dependency to support streaming of multi-file uploads.

This branch does not include any new unit tests for this new functionality, as it will start being exercised via the REST API integration tests once we integrate the solutions for #694 with #658. However, in the interest of verifying this new feature for the client works, I created this branch, https://github.com/usnistgov/dioptra/tree/test-client-uploading-support, that adds a simple /echoes endpoint that allows you to upload files, and the server echoes back the name and size of the files you upload. I tested it locally and I also created two simple REST API integration tests based on this endpoint to validate the test client implementation. Both seem to be working as expected.

Todos

• Add a utility that scans a directory, either with or without recursion, and builds up a list of DioptraFile objects for upload. The filenames will retain relative filepaths with respect to the source directory's root path, and be standardized in the POSIX format.
• Add a utility that accepts a filepath/list of filepaths and converts them into a DioptraFile object/list of DioptraFile objects to upload. The filenames will be stripped of any directory paths.