Implement server providers

 * Add new "provider" package
 * Rename former server "origins" into "providers"
 * Make catalog record channels use pointers
 * Move connector query types to "types" package
Implement server providers

 * Add new "provider" package
 * Rename former server "origins" into "providers"
 * Make catalog record channels use pointers
 * Move connector query types to "types" package
Merge branch 'providers' of github.com:facette/facette into providers

Conflicts:
	pkg/catalog/catalog.go
	pkg/connector/rrd.go
	pkg/server/library_graph.go
	pkg/server/worker.go
Fix missing `connector` package in library graph
Rename remaining originWorker references
Unexport function `stopProviderWorkers()'
Define explicit Connector type for connectors
Update example configuration files with providers
Remove remaining origin workers references
Update connector reference in library graph
Remove stack from library structures