Add function to ProjectType to get supported source types.

Required for https://github.com/zanata/zanata-server/pull/400
You can use
```java
ImmutableList.builder().addAll(XML_DOCUMENT_TYPE_DEFINITION.getExtensions()).addAll(...).build().
```

Or define a multimap in DocumentType. Multimap.values() will return a collection of all values.
```java
ArrayListMultimap<DocumentType,String> fileTypeMultimap =
                ArrayListMultimap.create();
fileTypeMultimap.putAll(DocumentType.HTML,
                Lists.newArrayList("html", "htm"));
...
fileTypeMultimap.values();
```