Base FileLoader on UriLoader

removes some duplicated code.
Remove volley dependence in uri/string loaders

Also removes the now unused FileStreamLoader
Simplify StringLoader to take UriLoader

eliminates the weird dependency on UrlLoader in
StringLoader
Allow https uris in UriLoader
Add tests for string and uri loaders
Add default loader and support for drawable ids
getStreamOpener -> getStreamLoader
buildStreamOpener -> buildStreamLoader
Remove redundant casts in getModelFor in Glide
Add methods to set placeholders in Glide.load
Update some of comments for Glide.load