add notes for setting up dev environment
add `--nulls` option

Controls how check_graphite handles null data from graphite. if you
pass `--nulls critical` then any null data will be considered a nagios
critical state. On other value for `--nulls`, null data will be
considered a nagios unknown state.

This is useful if you're using a graphite metric to determine whether
a host is up or down.

* defaults to unknown
* add rspec tests
* update docs