File preview support added

- diffrent previews according to the media type of the file

- media type sent via rest api next to other file properties

- read-only access only for now even for private files

- supported media types
  - text/plain
    - displayed simply in 'pre' html tag
  - source code files
    - displayed in 'pre' html tag with syntax highlighted
      using third-party library CodeMirror

- bower used
v0.1.9 package.json updated