New JsonReader class

This class aims to "fix" JSON parsing when the input contains trailing input,
for instance:

    []]

In this case, Jackson will happily read [], but ignore the trailing ]. This
class will not ignore it given the appropriate constructor arguments and will
throw an exception instead.
Rename JsonReader to JsonNodeReader, rename method

Also add a method to read from a Reader.
JsonNodeReader: add constructors
Make JsonLoader use a JsonNodeReader; update javadoc accordingly
Javadoc for JsonNodeReader
JsonNodeReader: javadoc update