Merge branch 'release/v0.3.1'

* release/v0.3.1: (61 commits)
  fixed file entry error at open recently files
  #29 help menu, export to html
  disable copy action in viewer context menu
  disable select all in context menu
  #23 enhancement context menu
  remove console.log
  update version info
  pageX, pageY -> clientX, clientY
  update techincal stacks
  fixed application default title on linux
  setup default application title
  drop drag&drop feature, as_desktop:false for linux
  fixed build error by js-url
  dragdrop event or help menu action
  implementation help menu action
  sanitize true -> false
  fixed #17 - conflict require() in require.js + node-webkit
  add Acknowledgements
  add markdown syntax
  impletmenting help > about action
  ...
fixed startup error #33
Merge branch 'hotfix/error-startup' into develop

* hotfix/error-startup:
  fixed startup error #33