Update README, convert to Markdown

More comprehensive and better-formatted main README.md, with begging slots
for Windows docs.
README.md: extra github flavour

Correct misformatting.
Fix build when using clang on Linux

The build environment was assuming gcc on the "linux2" platform, or at
least it was using "-z origin" when the linker was being invoked via clang
wrappers. Replaced with passthru variant for better portability.

We should be testing this to help OSX users and porters (see #49). The
current way of compiling with a nonstandard compiler is via the standard
environment vars CC and CXX:

   $ CC=clang CXX=clang++ scons

Builds with clang 3.4 and gcc 4.9.0 under Debian testing/unstable.
Better handling of optimization flags for lib/

Rethink the way scons should reinstate -O0 if compiling with debug=1.
Trying to be clever with CCFLAGS was causing problems: see #52.
Submodule bump
Fix test perms, allow nosetests from toplevel

Make test_compositeops.py executable, to avoid it being run by default by
nosetests when run at the top level. It only executes correctly when
compiled from a debug build.