Allow resources to be marked as transient
Make command implementations return the output resource twice
Make the command implementations return a single object
Mark input as transient on move

* Pushed down stat! method from runnable to transport
* Made transport mark resources transient on move
Mark the input as transient if delete_source option is set to `true`
Return the resource size instead of trying to modify it
Remove property type of agents
Allow definition of notifiers at project level
Add the `notify` verb to the projects DSL
Remove failing spec
Remove unnecessary (and failing) spec
Remove explicit project constructor
Execute notifiers after the project's pipeline

* Execute notifiers
* Use last_output instead of last_product as results of shells
executions
* Use both the product and output of transports
* Improved coverage
Make the ERB extension implicit
Report execution stack in a format adapted to notifiers
Add a base class for notifier implementations
Define a notifier metadata
Make shell pass the report inherited from project when pipelining commands
Fix validation of transport type
Provide formatting helpers to templates