#! (shebang) support

This update adds support for "#!/usr/bin/env snabb" scripts. This makes it easier to write ad-hoc snabbswitch code in separate files that don't have to be a part of the build.

Example:

    #!/usr/bin/env snabb
    c = config.new()
    ...
    engine.main(c)

This is achieved with two changes:

1. `snabbswitch` renamed to simple `snabb`.
2. `make install` rule that installs as `/usr/local/bin/snabb`.
3. New command-line syntax that accepts the full path to a script file (instead of Lua module name).