Suppor for specifications with Pod projects

This patch as it is looks good and I consider it acceptable to be merged.

Before doing that I'm looking for an option of the @CocoaPods/core team regarding:

- Enforce one project per spec/subspec.
- Enforce in the linter the presence of either build settings and file patterns (traditional way) or a project and target.
- Also I'm not sure about the syntax:
  - `xcodeprojs` could be project
  - it would be cleaner to have just `project` and `target`. If one is specified also the other must be. This can be enforced in the linter.