Add compile(Reader) method

JavaEngine provides the possibility to evaluate Java code from a Reader,
sometimes one might not want to have that code immediately executed.

For a Java File, the compile(File,Writer) method was provided already,
this commit adds such a method for compiling from Reader.
Add compile(String) method

A convenience method to compile from String, analog to eval(String).
compile(File, Writer) now uses Context error Writer

compile(File, Writer) is now compile(File) since we now use the
Context error writer, also this method now returns the compiled
main class for consistency.

A makeClassLoader(MavenProject) method was added, since this code was
used in compile(Reader), compile(File).