Use interpreter.n def instead of n val for replies

It may happen that execution counter is changed (usually reset)
within execution handler (e.g. %reset), so lets consistently
maintain its appropriate value.
Add %classpath magic
Simplify and sanitize interpreter initialization
Temporarily disable built-ins
Don't reset interpreter in %update if it wasn't inited yet

This is a little fragile, but allows us to configure dependencies
without awkward, early interpreter reset, e.g.:

In [1]: %libraryDependencies += "org.parboiled" %% "parboiled" % "2.0.1"

In [2]: %update
[info] Resolving org.parboiled#parboiled_2.11;2.0.1 ...
[info] Resolving com.chuusai#shapeless_2.11;2.0.0 ...

In [3]: import org.parboiled2._

In [4]:

Notice that line numbers didn't start from 1 after %update.
Add tests for the display framework
Add library dependencies to Display.ipynb
Ignore .ipynb_checkpoints in the entire tree
Add PNG display for java.awt.image.RenderedImage
Add JFreeChart plotting example to Display.ipynb