Add options to PDF

Hi,

First of all, thanks for the great work that you've put into SlimerJS.

I couldn't be more excited when PDF rendering came out, unfortunately it does not honour width and height.

Looking at SlimerJS code, I realised it creates a print profile every time you render to PDF.
I have found all available print settings here: http://dxr.mozilla.org/mozilla-central/source/widget/nsIPrintSettings.idl

Having that said, I've tweaked the code to accept dimensions as well as ability to set margin and resolutions.

Dimensions must be passed as millimetres, and resolution in dpi.

This solves #162.

The options are:
```
printSettings.marginTop
printSettings.marginRight
printSettings.marginBottom
printSettings.marginLeft
printSettings.unwriteableMarginTop
printSettings.unwriteableMarginRight
printSettings.unwriteableMarginBottom
printSettings.unwriteableMarginLeft
printSettings.resolution
printSettings.paperWidth
printSettings.paperHeight
```

Also, I've bumped to gecko 30.0. I've got some really heavy html that loads heaps of SVG. I believe previous versions had memory leaks, and randomly crashed with  `Illegal instructions` and `Segmentation Fault`. After updating, all errors have been solved apparently.

Please instruct me if you require anything else for merging this pull request, and hope it helps other developers!

Cheers
@bernardeli

it seems this variable is not used ;-)