Added memzip filesystem support for teensy

You can now append a zipfile (containining uncomressed python sources)
to the micropython.hex file.

Use MEMZIP_DIR=directory when you call make, or set that in your
environment to include a different tree of source files.

Added sample /boot.py, /src/main.py, /test.py and /src/test.py files.

Added run command so that you can execute scripts from REPL (until import is implemented).

Added build directory to .gitignore