Add minimumCapacity, remove ambiguous operator

This PR removes the + operator which is already provided by _ExtensibleCollectionType (SetTests.swift would not compile for me with Xcode 6.1 GM) and adds some methods from Dictionary (isEmpty, initializer with minimumCapacity).
Could you [omit the `get {` and `}`](https://github.com/github/swift-style-guide/blob/master/README.md#prefer-implicit-getters-on-read-only-properties-and-subscripts) please?