Adds a way to use folders in the migrations dir

We were starting to get a lot of files in our migrations dir, so we created a way to use folders in the migrations directory.

You can now organize your files like so:
```
<migrations-dir>/20140327123829-root-folder.js
<migrations-dir>/v1.0.0/20140327123829-test.js
<migrations-dir>/v1.1.0/20140327123829-test.js
```

Doing `db-migrate create v1.0.0/test` would result in a file created at `<migrations-dir>/v1.0.0/20140327123829-test.js`.

Running the script will make an entry in the migrations table for `v1.0.0/20140327123829-test`.

No support provided, if you have a need for it, feel free to use it.