Support DiskStorage in multer

[helje5]

As per 0.8.0 / Issue #7 we now support multipart/form-data uploads "multer style". 0.8.0 only supports the MemoryStorage though, DiskStorage is prepared but not implemented.

Adding DiskStorage needs a little refactoring to support proper async writing to disk and all the back-n-forth due to this. The multer DiskStorage also has async callbacks to decide on storage names and such.

To implement this, the multer.Context needs to provide the DiskStorage a place to store processing state, i.e. what async operations are still running and need to complete before next can be called.