frontend: skip task creation at build trigger times w/o task_frontend config

Skip build task creation at build trigger time, if task_frontends have not
been configured. Facilitates easier unit testing.
distcilib: implement job & build related requests
frontend: test: fix naming in sample job configuration
frontend: constants: add missing entry for artifact read error
build: add all distci python files for pylint target
worker: workspace operations
worker: first version of git checkout worker
worker: first version of execute shell worker
worker: first version of publish artifacts worker
worker: first version of build control worker
build: package the new workers into the resulting EGG
worker: remove the example calculator worker
Merge pull request #8 from hnousiainen/master

basic workers