feat(style.md): first cut at style guidelines
feat(list.lean): start on list library
feat(equality_test.lean): experiments with equality axioms

This reduces most of the current axioms for equality and heterogeneous equality to the basic rules for Id as an inductive type, plus Pi injectivity. There are universe problems that should go away in Lean 0.2.
feat(todo.md): added to do list
chore(list.lean): added copyright info to header
Added natural number libraries using inductive types with add and mul
chore(.travis.yml): set up travis-ci for backup and diff notification