Internal configuration for global defaults

Allow global ZeroClipboard DOM attributes to be more configurable. This is a continuation of #412.

Creates an `_internalConfig` object, which reads keys from `package.json`, to configure DOM attributes used throughout the library.

These values cannot be changed at runtime through ZeroClipboard's config API. Instead, they are used during the build process to generate the JS and SWF components of ZeroClipboard.