Add doctype to preview template

Adding <!DOCTYPE html> because I was getting the browser user agent styles applied instead of the DCC styles over in /multidevice.

StackOverflow: http://stackoverflow.com/questions/11348727/why-does-user-agent-stylesheet-override-my-styles

Who knows what other formatting issues it may be causing.
Merge pull request #115 from pearlchen/preview-template-fix

Add doctype to preview template