Correction of the JDK detection on Mac OS X

Java homes on Mac are under Contents/Home
Code style enhancement

* The script stops if it is executed with Python 2, printing a pointer
  to the jPype1 GitHub page (https://github.com/originell/jpype)
* NoJDKError now contains all tested folders (including JAVA_HOME)
* JDK is now searched outside the "except" block, which is prettier
Modified libjvm file lookup

Instead of just walking through the Java home folder, jPype now looks
for a file which under one of the following folders, in order of preference:

1. server (standard for Java 64 bits) -> preferred one
2. client (standard for Java 32 bits) -> only works with 32 bits versions of jPype
3. cacao (found on Ubuntu)
4. jamvm (found on Ubuntu) -> seems to crash jPype