Allow convertion into Ulysses texbundle format

Each textbundle consists of three components:

1. info.json: it contains the metadata in the following form:

{
"creatorURL" : "file:\/\/\/Applications\/UlyssesMac.app\/",
"transient" : false,
"type" : "net.daringfireball.markdown",
"creatorIdentifier" : "com.ulyssesapp.mac",
"version" : 2
}

2. text.md, in the following form:

\# Untitled Note

\![][image-1]![][image-2]

\[image-1]: assets/Picture.jpg
\[image-2]: assets/Picture-1.jp

3. optionally an assest subfolder that contains the assets fitting to the relative path in the text.