Move selector creation methods onto new Selectors class

Previously the helper methods for creating Selectors were on the
reactor.function.Functions class. This wasn't the ideal place for
them as Selectors aren't necessarily related to a functional style of
programming.

The helper methods have been moved to a new class:
reactor.event.selector.Selectors. In addition to the existing very
short methods, longer more discoverable equivalents have been added:

public static Tuple2<Selector, Object> anonymous()
public static Tuple2<Selector, Object> $()

public static <T> Selector object(T obj)
public static <T> Selector $(T obj)

public static Selector regex(String regex)
public static Selector r(String regex)

public static Selector type(Class<?> supertype)
public static Selector t(Class<?> supertype)

public static Selector uri(String uriTemplate)
public static Selector u(String uriTemplate)

While the unconventional (and probably always controversial) $ method
remains, the other short-hand methods (R, T, U) have been lower-cased
to be somewhat more conventional.

Fixes #127
Remove Fn, P, R, and S alias classes

The Fn, P, R, and S classes are simple extensions of Functions,
Promises, Reactors, and Streams respectively. They exist solely to
provide shorter-named equivalents. There are two main problems with
this:

1. They are all in the same package, even though the classes that
   they extend are, quite rightly, spread across three separate
   packages
2. The names, while short, are not concise: they convey almost no
   meaning. If you're not familiar with Reactor you'll have no idea
   what the class is or does. Even if you are familiar, you may still
   be confused. For example is S short for Streams or Selectors?

The first problem could be addressed by moving the classes alongside
their longer, more informatively named equivalents. However, that
would not address the second more fundamental problem. This second
problem leads to code being hard to read unless you're intimately
familiar with the Reactor API. To avoid this problem, the alias
classes have been removed.

Fixes #129