unify/clarify/simplify makefiles

- there is now only one Makefile, at top level
- it knows how to build pkg/python and cmd/go-python, but not
  tests (the old Makefile didn't work)
- simplify Python code to query distutils
- change so "make" output just shows CGO_* values, not the big hairy
  "python -c" command lines, eg.

    $ make
    (cd pkg/python && CGO_LDFLAGS="..." CGO_CFLAGS="..." go install -compiler="gc" .)
reorganize code to make "go get" and friends happier

The import path is now "github.com/sbinet/go-python".