Add Makefile to automate common development tasks.
Add Makefile to automate common development tasks.

This Makefile is mostly for development. For example, you can
run the unit test as below:

    $ make test
    /usr/bin/env python -m unittest discover -v test
    test_parse (test_formula.TestFormula) ... ok
    test_safe_eval (test_formula.TestFormula) ... ok
    test_tokenize (test_formula.TestFormula) ... ok

    ----------------------------------------------------------------------
    Ran 3 tests in 0.001s

    OK

xica/formula