Inmemory

I took care of Issue #20 for you. This adds an `inMemory` option to Multer. A file.buffer property holds the data in-memory and doesn't write it to disk. Note that when inMemory is true, `dest` is ignored and file.path is null. When inMemory is false, file.buffer is null. Also updated mocha tests and documentation. 

Let me know if I need to tweak anything.