Cedar-backup3

* Fix incorrect exception raise without % in util.py, found by accident.
* Fix cut-and-paste typo 'iplemented' in cli.py, config.py, and util.py.
* Fix bugs in the ByteQuantity changes from v3.1.0, so comparisons work properly.
* Adjust amazons3, capacity and split to use ByteQuantity directly, not bytes field.

* Enhance ByteQuantity so it can be built from and compared to simple numeric values.
* Improve the way the amazons3 extension deals with byte quantities.
- Fix configuration to support quantities like "2.5 GB", as in other extensions
- Improve logging using displayBytes(), so displayed quantities are more legible

* Improvements based on integration testing with my own backup configuration.
- Fix problems with pickle by using binary open, protocol=0, fix_imports=True
- Be stricter with the way files are opened/closed, relying on the new 'with' idiom
- Make sure every file opened for use with executeCommand() uses mode "wb"

* Create project in Mercurial at BitBucket, alongside Cedar Backup 2.
* Convert to Python 3, using v2.24.2 as the basis for conversion.
* Rename files (logs, config, executables) to use cback3 prefix rather than cback.
* Remove support for Windows and Cygwin, which was never advertised and rarely tested.
* Fix a variety of minor warnings from pylint 1.4.4, most of which also appeared in 2.24.2.
* Clean up manpages and add notes about migrating to version 3.
* Review user guide, fix broken links, make minor tweaks to wording, etc.
* Fix long-standing bugs with pre- and post-action hooks, ported from v2.24.4.

* Add optional size-limit configuration for amazons3 extension.

* Implement a new tool called cback-amazons3-sync.
* Add support for missing --diagnostics flag in cback-span script.