Feature: permanent whistle

[vadymmarkov]

@RamonGilabert @zenangst @onmyway133

BREAKING CHANGE!!!

1. duration property has been removed from Murmur.
2. Whistle functions has new attribute action to show either temporary or permanent message:

let murmur = Murmur(title: "This is a small whistle...")

// Show and hide a message after delay
Whistle(murmur, action: .Show(1.5))
Whistle(murmur) // Show(1.5) is by default

// Present a permanent status bar message
Whistle(murmur, action: .Present)

3. Whistle message could be hidden with a new Calm function:

// Hide a message
Calm()

4. Action is renamed to WhisperAction because we got WhistleAction.

[zenangst]

[zenangst]

@vadymmarkov you should probably do something about them tests.