[TIDOC-1080] Support exclude-platforms key

TESTING DIRECTIONS:

1. Edit the YML files to use the `exclude-platforms` tag.  This will generate a list of supported platforms for an object's method, property or event by using the object's `platforms` key (or the default platform list if this is not defined) minus the ones specified with the `exclude-platform` key.

For example:

If an object supports all platforms, changing an attribute with:
```
    platforms: [iphone, ipad]
```
To:
```
    exclude-platforms: [android, mobileweb, tizen]
```
Produces the same result.

If both `platforms` and `exclude-platforms` are defined, `platforms` is used not `exclude-platforms`.

If the `exclude_platforms` method returns an empty list, a warning is outputted to the terminal since this should not happen and means the object's attribute is not supported on any platform.

2. From the `apidoc` directory, execute:
`validate.py` --> make sure syntax is correct
`./docgen.py` --> generates HTML version in `/dist/apidoc`
`./docgen.py -f jsduck -o ../dist`  --> generates `dist/titanium.js`
`./docgen.py -f jsca -o ../dist` --> generates `dist/apidoc.jsca`

3. Inspect the files to make sure the correct platforms are generated. Note that the jsca version does not list the platforms for events.