Holder: SS4 compatibility

[robbieaverill]

This is a holder issue for upgrading to SilverStripe 4 compatibility.

Pre-requisites

• [x] branch current master off for maintaining the current version
• [x] update composer branch alias

Tasks

• [x] update composer constraints for 3.x releases
• [x] namespace classes and implement core namespaces - include renaming folder structure for PSR-4 autoloading, and including an .upgrade.yml map for the upgrader tool
• [x] update configuration that requires namespacing
• [x] rename code to src (Separate from any PSR-2 changes so as to maintain git history)
• [x] update tests
• [x] PSR-2 compat if not already implemented (doesn't look too bad)
• [x] update Travis config to build PHP 7.x and minimum 5.5
• [x] update documentation as necessary
• [x] change module so that it gets installed into vendor folder
• [x] update translations and ensure translations are updated on transifex
• [x] define table names for datamodel classes

Extra

• [x] update Guzzle to latest version that still supports PHP 5.5 (guzzle/guzzle is abandoned) ~3.9 is required by omnipay/common - can't change
• [ ] throw in any breaking changes that are waiting in the wings :-)
• [x] Integrate SS4 logging (via monolog)

[bummzack]

Sweet, thanks Robbie