Support for plugins to add multiple paths at once

[tinproject]

In #68 it shows the need for plugins to add multiple paths at one time.

I'm currently implementing a Apispec.add_paths() function, but I don't feel this is the better way to do this.

I've thinking that allowing path helpers to return a list of Path objects, in addition to Path object, is better and easier. Perhaps refactor the code that add a Path object to the spec to an internal method and leave in add_path() the code to extract the Path objects and pass it to the internal method.

@sloria What do you think?