Rename Tag#key to #tagname.

This way the class attribute @tagname corresponds to :tagname field
of the records produced by #parse_doc.
Rename :key to :tagname in signature data.
Extract Docs.view.Signature class.

The signature rendering code in frontend was duplicated in three
separate places. Now at one.
Wrap signature labels inside additional span.

First step in generalizing the styling of these things.
Make signature styles easily overwritable.

One can now style his own signature label by just specifying in his
additional stylesheet:

    .signature .my-tag-name {
      color: red;
    }
Add Tag#css attribute for injecting additional CSS.

For now the CSS just gets added to index.html file, next step would
be to move it into a separate file.
Move all signature styles to Tag classes.