v1.14.0 ResumableUpload changes

Feature added:

• #749 Resume upload after program restart.

Bug fixes:

• #755 Incorrect resume behaviour if first upload chunk fails.
• #602 Resume fails with non-seekable stream when last upload chunk fails.
• #767 Invoke event handlers safely and consistently.

Test changes:

• #735 Rewrite MediaUploadTest to use test HTTP server. However, this fails on Travis due to a mono bug, so the original MediaUploadTest is still in use.
• #732 Add a MediaDownloader test with an empty response body.