Use 4*1024*1024 to set chunked_size
Rename method parameter to chunk_size
Add strip_path! helper method
Add default upload behavior

When uploading local files to Dropbox that already exist remotely, we'll
not overwrite or upload (causing a duplicate file) by default.

The reasoning behind this default is that it does the least amount of
damage. That is, if we used 'cp' as a blueprint; we would end up wasting
bandwidth by overwriting files already at the remote location. The same
goes for duplication.

Instead we'll add flags for those behaviors, giving the user the option
to determine what best suits the occasion.
Merge pull request #11 from bmichelsen/default-upload-behavior

Default upload behavior