cylc prerequesites: ignore pre-initial cycle tasks

Addresses #541

This change causes tasks to ignore dependencies on tasks with cycle times prior to the initial cycle time e.g. `a & b[T-6] => c` on the first cycle becomes: `a => c`. In doing so it adds a object/library for cleaning conditional prerequisites of particular entries.

As a result of this change test `Quickstart/a/` from the test battery will fail since the triggering is based on a task having not run due to a dependency prior to the initial cycle.

@hjoliver - please review. Any ideas on what you'd like changing about the quickstart/a/ test? I couldn't see a trivial way to adjust it - played with the idea of `Model[T+6] => Model` but obviously this won't work as there are multiple cycles and the triggering task only triggers `Model` when run in cycle 2011010112.