Add --rebuild option to
Add Rake::Pipeline::DynamicFileTask

This is a WIP adding support for Rake tasks with dynamic dependencies.
Yehuda's original implementation is here:
https://gist.github.com/806d2028b011d36fb974

Ultimately, this will be integrated into Filter#additional_dependencies
so filters will be able to use it, but it's not there yet.
Add json dependency
Add DynamicFileTask#has_dynamic_block?
Don't try to read the mtime of nonexistent files.
Adapt Rake::Pipeline::Filter to use DynamicFileTask
Update some docs
Create tasks for additional dependencies
Only calculate dynamic prerequisites if the task has a dynamic block
Minor refactor
Cleanup specs a bit
Exclude the current task from dynamic prerequisites
Add spec for dynamic test manifest checking
Move the manifests from Rake::Application to the Rake::Pipeline::Project class
Add Pipeline#project
Move manifests from the Project class to instances.
Travis won't let me make files from the year 3000.
Merge branch 'dynamic_task'
Add a getting started guide
Add a minifying files example