[python] fix get_library()

Before this fix, the LLVM Python bindings on SVN trunk always fail with:
	Exception: LLVM shared library not found!
since it's still looking for a library named "LLVM-3.1svn".

Besides updating the LLVM version in the library name,
this patch also changes llvm.get_library() to make it possible to run
the unit tests without installing the LLVM shared library into a
default linker search path.

e.g. after this patch, running the llvm/python unit tests with:
LD_LIBRARY_PATH=../build/Debug+Asserts/lib nosetests -v bindings/python/llvm/tests/
would work on Linux.

Patch from Scott Tsai (with some minor modifications)

Patch also acked by Gregory Szorc

llvm-svn: 168390