Only put regular files

You should get an error message when you try to `put` directories, etc:

```
$ ./bin/doss.sh put ~/Desktop/files/*
ID	Filename	Size
1	/Users/hrowland/Desktop/files/IMG_1579.JPG	452.1 KB
2	/Users/hrowland/Desktop/files/flame robin.jpg	24.1 KB
Aborting, rolling back all changes...
doss: not a regular file: /Users/hrowland/Desktop/files/zoo-pictures
```