Add support for multiple positional path arguments

[guykisel]

This replaces #68. Instead of modifying Prospector to support multiple paths, I've modified prospector.run.main to create a new Prospector instance for each input path.

Detailed changes:

• If more than one path is provided and one of the paths is a directory, raise a ValueError
• Explain multi-input directory restriction in --help
• Add Path to each summary
• Don't print Messages header if there are no messages

[coveralls]

Coverage decreased (-0.15%) when pulling 56ce6fa on guykisel:multi-path-input into 1aaa0d6 on landscapeio:develop.

[carlio]

Thanks! In the end I had to jiggle things around a bit to get it to work with the configuration changes I had made elsewhere, but actually it ended up being a bit simpler - the Prospector object can now handle being given multiple paths instead of having to create a new object each time.

[guykisel]

Nice! Thanks.