Enhancement: File Download Functionality

- Adds file download functionality by streaming the file from Deuce
--- We can later add the option to only download the required blocks; but this provides good testing as it is
Bug Fixes

- Pep8 compliance
- Updated message in download file failure to reflect the difference in system operation
Bug Fix

- Fixed DownloadFile() so that it returns True on success as documented
- Fixed download rate in DownloadFile
--- this does result in a rediculous transfer rate for small time interfaces; but is more accurate compared to the previous calculation
Bug Fix and Tests

- Bug Fix: Don't include the time to allocate and initialize downloaded_bytes in the time for the download (minor difference, likely unmeasurable, but nonetheless the right thing to do)
- Bug Fix: Use total_seconds() for calculating the download rate, removes need to check that the time is positive, also returns a fractional number so produces a better result
- Tests: Added test for the deuceclient.client.deuce.DeuceClient.DownloadFile
Bug Fix - Re-enable a try/except block around it
Merge pull request #31 from BenjamenMeyer/enhancement_file_download

Enhancement: File Download Functionality