Refrapt

Attempt to address 26 by allowing for retries when a server becomes overloaded. Retries are staggered to help decrease load.

Also fix a small issue when default settings are None were considered as strings.

A minor release only.

Bugs fixed in this release:
- 25 Refrapt crashes in Test mode

**Bug Fixes**

- Address issue 23 by ensuring the same URI is not processed multiple times during the clean process (caused by trailing slash not common to all repositories from the Configuration file).
- Ensure download URL is formatted using forward slashes
- Ensure download command only adds custom arguments if provided

**Optimisations**
- Only attempt to download non-latest files, rather than all of them (saves Wget the trouble of checking the files)

Includes the following fixes:

- Ensure that repositories exist by checking whether Index files were downloaded before proceeding (21)
- Allow for leading whitespace in front of `set` variables in the configuration file (21)

This release adds a new feature, Clean mode, which can be enabled via the Command Line option `--clean`. This will skip the normal processing, and evaluate the existing mirror stored on the machine, and clean up old files that aren't necessary.
This allows for running Refrapt as a cronjob in order to only clean mirrors.

Issues addressed in this release:
1. Add Clean Mode at request of 17
2. Fix bug identified in 18

Adds a new option to the Configuration file called `disableClean` which when set to True will skip the cleaning process entirely, rather than requiring that the clean option is disabled for each individual mirror.

Goes towards 17.