Set correct HTTP User-Agent for outbound requests

[smn]

• [x] Include some name
• [ ] Include some version number that reflects the build
• [x] Include some way of pointing at this code base / documentation

What we have now is not of much use.

[hodgestar]

This was partially addressed by #37. User-Agent is now: https://github.com/praekelt/mama-ng-scheduler/blob/69fec00147b45a35c75a9ad9fa6109267834014d/grails-app/services/mama/ng/scheduler/HttpRequestService.groovy#L21

[hodgestar]

What's left to do is to pull in the application version dynamically from config / build information rather than statically including it.