Add scriptable components support

[mattgarrish]

Another edupub satellite spec we need to check:

1) that only one xhtml document ("base document") is referenced from a child link (spine) of the scriptable component collection

2) that the base document is referenced in the content from an iframe/@src

3) that the base document includes scripting necessary for communication (no idea how this is verifiable beyond the document being scripted)

4) that the base document does not attempt to access window.parent properties (if verifiable)

5) that an epubsc:version property has been set, whether the epubsc is packaged or embedded. Value must follow the pattern: [0-9]+(.[0-9]+){2}

6) warn if the scriptable component's resources are not stored in the path /components/ (where is the same value as the first dc:creator).

7) all IDs should be unique, but no explicit definition of uniqueness is given so this may be untestable

8) and, less importantly, verify the values of the other three epubsc:* properties: storage-required (true or false), network-access-required (true or false), required-params (name=type, as defined in spec). Should generate an error if other epubsc:* properties found.

[rdeltour]

Basic support has been added, but needs review and additional. At least, valid Scriptable Component content should no longer be rejected.

I'm removing the 4.0 milestone and will keep this open while the spec matures to Recommended Specification.

[tofi86]

Removing from 4.1.0 milestone. Feel free to work on it and assign it back to 4.1.0 as long as we're still working on it.