Fix extensions outputs

• change output for union extension from Property to MutableProperty;
• change output for mapper extension from Property to MutableProperty;

New extensions

• add plus-assign operator to CompositeSubscription;
• add union function for properties;
• add mapper function for properties;

Primitives support

Now you can use Properties for such primitives as Int, Long, Double, Boolean without boxing.

Replace getter and setter functions with Kotlin's property

Warning! Breaking api release.
Getter and setter functions was replaced with Kotlin's property.
Was:

val property = propertyOf("hello)

property.get() // "hello"
property.set("world")
property.get() // "world"

Became:

val property = propertyOf("hello)

property.value // "hello"
property.value = "world"
property.value // "world"

First release of properties

Merge pull request #1 from green-nick/dev

Dev