UI refinement for LetsHire

Refine the UI of LetsHire,  please refer to the story of [final UI refinement for LetsHire](https://www.pivotaltracker.com/story/show/51699119), [decide on the LetsHire Logo](https://www.pivotaltracker.com/story/show/51076989)
The detailed information is listed as below: 

1. unify UI styles and make it look consistent, including: header, footer, button, dialog, etc;
2. redesign login and admin initializing page;
3. redesign the layout and style of dashboard page;
4. remove the gradient surrounding table or other content and make it simple and clean;
5. clean up html and css codes and reduce the hierarchy of html, so make it more clear and understandable;
6. cleanup the alignment and spacing