Added 'Side effects occur again for each subscription' guideline
Merge branch 'design-guidelines' of https://github.com/ReactiveCocoa/ReactiveCocoa into design-guidelines

Conflicts:
	Documentation/DesignGuidelines.md
Updated 'Side effects occur for each subscription' following PR suggestions
Merge branch 'design-guidelines' of https://github.com/ReactiveCocoa/ReactiveCocoa into design-guidelines
Merge branch 'design-guidelines' of https://github.com/ReactiveCocoa/ReactiveCocoa into design-guidelines
Clarified `RACSignal` example for 'side effects occur for each subscription'
Clarified description of side effect behaviour in `RACStream` operations.
Fixed typos and some clarifications for 'side effects behaviour'
Typos
Merge branch 'design-guidelines' of https://github.com/ReactiveCocoa/ReactiveCocoa into design-guidelines
Added an additional example
Typos
Merge branch 'design-guidelines' of https://github.com/ReactiveCocoa/ReactiveCocoa into design-guidelines
Fixed some more typos
Rewrote first paragraph as suggested
Merge pull request #306 from EnthusiasticCode/design-guidelines

Content for 'Side effects occur again for each subscription' guideline