Implemented AsimovWebPackageSource

Its now possible to browse to a webserver to fetch packages from there.
Merge branch 'web-source' of github.com:thelalle/asimov-deploy into thelalle-web-source
fixed curly indenting on pull request, need to figure out how to best unit test or integration test the new AsimovWebPackageSource, but it goes in anyway :) thanks Lars Stenberg