New formatter to print steps catalog

A new formatter class 'StepsCatalogFormatter' is added. The formatter is
named 'steps.catalog'.

It is intended to provide a catalog of available steps to non-technical
users, so it prints the steps as they would appear in the feature file
and ommits the step functions' names and locations.

A feature file with a test case is also provided
Convenience command line option '--catalog'

This convenient command line option shows the catalog of available step
as formatted by the 'steps.catalog' formatter.

This option is a short cut for '--dry-run --no-summary --format
steps.catalog'
Use new command line option in test case

The new '--catalog' command line option is used
Updated formatter.help test case

The Help Formatter feature was updated to support the new steps.catalog
formatter
pull #271: Provide steps.catalog formatter
Merge branch 'step_catalog_formatter' of https://github.com/berdroid/behave into berdroid-step_catalog_formatter
Pull #271: Provide steps.catalog formatter and --step-catalog command line option.
Merge branch 'berdroid-step_catalog_formatter'
Comment for pull #271 (steps.catalog formatter).