Misc fixes

Here's a few fixes and improvements we've made to get it working for BugHerd:

Depends on the linting and standards pull request (already merged into master): https://github.com/niklasvh/html2canvas/pull/100

Changes:
- Background color will be white if none or transparent was set
- Can now check origin from other windows using the new `windowHref` option
- Images will cache in memory

Usage for `windowHref` would look like this:

```
window.html2canvas(window.parent.document.body,{
          windowHref: window.parent.location.href,
         // ...
```