Significant cleanup in api, and more importantly in sementics, of SkFontHost

Significant cleanup in api, and more importantly in sementics, of SkFontHost
- FindTypeface is now CreateTypeface, and what it returns is always considered
    a new instance, even if it is from a cache (in which case it will have been
    ref()'d. The caller must always balance its ownership by calling unref() on
    the result when they are done.
- CloseStream is gone, since the caller can/must call stream->unref() when they
    are done using it.
- ResolveTypeface is now ValidFontID, and just returns a bool.

git-svn-id: http://skia.googlecode.com/svn/trunk@109 2bbb7eff-a529-9590-31e7-b0007b416f81