Implement Dropbox support

This will bring in support for Dropbox into the library, i.e.:

• Have Dropbox*Job implementations for all defined types of jobs.
• Support Dropbox in the Directory synchronizer.

Progress:

• Boilerplate code
• DropboxCreateDirectoryJob
• DropboxCreateDirectoryJob unit test
• DropboxGetFileInfoJob
• DropboxGetFileInfoJob unit test
• DropboxListFilesJob
• DropboxListFilesJob unit test
• DropboxDeleteJob
• DropboxDeleteJob unit test
• DropboxUploadFileJob
• DropboxUploadFileJob unit test
• DropboxDownloadFileJob
• DropboxDownloadFileJob unit test
• DirectorySynchronizer works with Dropbox
• Make sure we check all errors as described here

Closes #22 (closed)