Implement methods for finding all targets accessible from a source and all sources accessible from a target.
Tests for lookup_target, lookup_targets, lookup_source and lookup_sources.
Add tests for the add_entry, remove_entry, remove_connector, remove_conversation and remove_transport_tag.
Add tests for add_oldstyle_conversation.
Simple tests for transitive_closure and reverse_transitive_closure.
Add __init__ for account tests package (how was this not here before?).
Better names for transitive lookup methods (they are now called transitive_targets and transitive_sources respectively).
Give parameter types.
Re-write transitive_sourcs and transitive_targets to handle routing blocks (and actually all other routing table entry types) correctly.
Add tests for flip_direction (was flip_router previously but flip_direction seems like a better name).
Only include the appropriate router direction when adding routers to results in transitive_sources and transitive_targets.
Explicitly set the locmem email backend for tests to avoid cases where it somehow fails to be set correctly in tests (review by @justinvdm).
Merge branch 'develop' into feature/issue-386-add-way-to-list-reachable-channels
Merge branch 'feature/issue-386-add-way-to-list-reachable-channels' into develop