Added support for Xcode workspaces when building plugins

Did this one for fun tonight. Any feedback appreciated. Some details:

- Tries workspace first, falls back to projet if *.xcworkspace* file is not found.
- By convention, the scheme to build must be named the same as the plugin itself. I saw some discussion in [pull request #69](https://github.com/supermarin/alcatraz-packages/pull/69) in the alcatraz-packages repo about supporting workspaces and the need to specify a scheme. I think starting with a convention like this makes sense.
- Indirectly allows using CocoaPods in plugins if one has the Pods directory under version control.