Project restructure, new build manager interface and internal build status API

[NiklasRosenstein]

• Project restructure
  • Create new core module with classes that manage processes, builds and scheduling
  • Move Flask application into server module
• flux_config now has a build_manager and scheduler option
• Added an internal build API

Todo

• Implement build sections (ie. named sections that envelop build script output)
• Implement build groups (a single ref should be able to kick off multiple builds, likely another PR though)