Target details cleanup

[Aurel300]

Closes #339, #425.

• A lot of cleanup in C++, particularly the Build.xml section; some sections (e.g. Thread and stacks) still need more work, the tone is more like an article/demonstration rather than a manual.
  • Removed the C++ pointers section - I don't know what to put there (I don't really understand the differences between Star, Pointer, Ref in the cpp APIs). It never had any content.
• Added very short getting started section for Neko and Python.
• Added a getting started section for HashLink, as well as a section about dual compilation.

Future work:

• Create a C++ pointer section.
• Unify the new getting started guides from haxe.org with the getting started pages in the manual.

[Simn]

I'm sure the community will review this over time. :P