Added logging. Now each module has it's own logger (_logger) which
the module can use to write log messages. In addition a file "driver/logconfig.py"
has been added which is imported by command line tools to configure the root logger
from which the other module logger inherit their settings.

The logging level is controlled by setting the WLLVM_OUTPUT environment variable.

Various bits of logging code have been added to aid with debugging.
Fixes for python3 compatibility. Still seems to be working fine for python2.7.
Remove duplication of ELF section name. It is now just defined in utils.py
Added argument parser (argparse) to extract-bc
extract-bc can now build LLVM bitcode archives from static libraries built with wllvm.
Handle environment variable LLVM_COMPILER not being set slightly better.
Tidied up the appearance of logging output. If WLLVM_OUTPUT=DEBUG a different format
is used that is much more verbose.
Added example of using extract-bc to build a LLVM bitcode archive