Remove +switch:cases:, change missing default behavior

A missing default now sends an error instead of throwing an assertion
failure.
Inline RACSwitchSignalExamples back into RACSignalSpec
Test that a missing case and default causes a signal error
Match nil values as RACTupleNil
Fix duplicate subscriptions in tests
Merge branch '2.0-development' into remove-switch-without-default
@joshaber can't handle the power of

THE OTHERWISE OPERATOR
Merge pull request #689 from ReactiveCocoa/remove-switch-without-default

Remove +switch:cases:, tweak +switch:cases:default: behavior