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Refactoring for #59
Ignore statedb
Remove unused variables

This removes two unused variables from `serpent.py`. In the first case,
we still need the call for `compile_expr(...)`, since the result value
is modified by that call. In the second case, the variable is not needed
at all.

Part of #59
Add entry-point in tools for serpent

This removes the `main()` method from the library part of `pyethereum/serpent.py` and
moves it into `tools/serpent_cli.py`. It also adds a console_script entry in
`setup.py`, so after installation, the serpent-CLI tool becomes available as `serpent`.

(If you're using buildout, this will simply be `bin/serpent`).

This is part of #59