graph: add some stubs
add example, fix error in topological sort
tests: moved example.go to graph_test.go
graph: throw error on creation if graph is invalid
graph: add GetDependants method to retrieve next set of jobs
graph: rename getDependants to jobsDependantOnSingleJob
graph/job: fix fmtpolice errors
Merge pull request #1 from colegleason/graph

Basic graph construction