Adds support for full page screenshots

By passing full=true as a query param this will allow for the full page to be captured rather than the window size.  Fixes #4
Major refactor to express

We're now using the `expressjs` framework to power the REST side of things.
webshot also allows you to define the dimensions of the screenshot you're
taking, so there is no (immediate) use for imagemagick anymore.
Merge pull request #6 from okfn/express

Add an index page with a simple form on it
Removed npm install from Procfile
[ux][m]: style front page and refactor form generation a bit to make ux nicer.
Merge branch 'master' of github.com:okfn/webshot
[#2,home][xs]: add github link to top navbar.
[#10][xs]: support ?url= on home page - fixes #10.
Add a delay of 2 seconds so AJAX heavy websites can run.
Send out a content-type attachment header with a prettier name
Merge pull request #9 from okfn/fileNaming

Better image naming (for download and drag-n-drop)
Make 'delay' an option
Merge pull request #13 from okfn/ajaxDelay

Add a delay so AJAX-heavy websites are fully rendered
Specify delay in ms with an upper limit of 10000