Usability improvements for driver

Several changes are now in place to make the driver more intuitive and easy to use:

- if the driver is run in help mode (or without any arguments), it now displays the valid commands it could successfully import, and then shows all commands that it couldn't import in a separate section. The original error message for why the import failed is also included.

- ```--verbose``` has been removed from the driver's special flags as we want users to see why configs can't be loaded.

- if the user supplies an unknown command name, the driver now prints an informative error message saying that it can't recognize the command. This message is distinct from the case where a valid command name is passed but the config can't be loaded.

Example output:

Config import error:

```
$ biom
usage: biom <command> [<args>]

The currently available commands are:
   add_metadata        Add metadata to a BIOM table
   convert             Convert to/from the BIOM table format
   show_install_info   Provide information about the biom-format installation
   subset_table        Subset a BIOM table
   summarize_table     Summarize sample or observation data in a BIOM table

The following commands could not be loaded:
   validate_table      Error: No module named dateutil.parser

See 'biom help <command>' for more information on a specific command. 
```

Unknown command name:

```
$ biom foo
Unrecognized command foo. Please make sure that you didn't make a typo in the
command name.

To see a list of all available commands, run the following command:

   biom

```

Fixes #164, #165.