added documentation and customization options

- added support for authenticated download
- added customization options
  *  shockBase - URL of the SHOCK server to interact with, default is config.js-shock_url
  *  width - width of the browser in pixel, default is 1200
  *  height - height of the browser in pixel, default is 600
  *  title - text displayed in the title bar, default is "SHOCK browser"
  *  showFilter - boolean whether the filter section is visible, default is true
  *  presetFilters - hash of field name -> field value that is always added to the filter
  *  enableUpload - boolean whether upload is enabled, default is true
  *  enableDownload - boolean whether download is enabled, default is true
  *  previewChunkSize - size in bytes that is loaded from the server for the preview of a file, default is 2 KB
  *  uploadChunkSize - size in bytes uploaded to the server per chunk, default is 10 MB
  *  currentLimit - maximum number of files loaded initially and whenever scrolling to the bottom
  *  detailType - one of [ info, attributes, acl, preview ] indicating the initial display when selecting a file, default is info
  *  authHeader - authentication header used when interacting with the server
  *  customPreview - optinally provided custom function for node preview, receives an object with the selected node (node), the first previewChunkSize bytes of the file (data) and the error if exists (error), must return the HTML to be displayed in the preview section