Use noCopy versions of NSString init in Base64 code.
Add an associatedObject field to CDVPluginResult.

Could have used objc_setAssociatedObject, but this seems simpler since
we own the class.
[CB-2652] Make FileReader.readAs*() functions run on a background thread

Also refactored to remove copy/paste.
[CB-2633] Add FileReader.readAsBinaryString()