Should syntax implementation

This pull-request allows to write specs in ```should``` style:
```swift
it("should work") {
  actual.should.equal(expected)
  3.should.beGreaterThan(1)
}
```

It provides extensions for all basic types, ```NSObject``` and some ObjC collection types (`NSSet`, `NSArray`) with ```should``` variable. 
Each of the extension supports using all available matchers of Sleipnir.