Revamp icons

> Add support for highlighted icons
Add non-retina versions for all icons
Add new icons for tag, fetch, pull, push, detail and tree
Add icons for collapsed and expanded folder items
Refine colors for file icons
Rename Images.pxm to Icons.pxm

Here is another revamp of the Gitx icons.

Along with it I've added support for custom images for highlighted items, this allows to use different colors to greatly improve the visibility, by taking the selected background color into account and adding a subtle drop-shadow.

This required some changes to how icon images are loaded. I removed the class static variable in the specific PBSourceViewItem classes (e.g. PBGitSVFolderItem) and replaced that with a generic implementation of `icon:` and `highlightedIcon:` in the PBSourceViewItem base class. Loaded icon images are configured to be [always cached](https://github.com/afh/gitx/commit/02ed90ae3e4263752c175a8d3aa8c8a6c4910c31#L2R147) and from my point of view this should have the same effect as the class static variable.

With this a PBSourceViewItem subclass simply returns the "base name" of its icon (e.g. `Folder`) in the newly added `iconName` method and the PBSourceViewItem takes care of adding the `Highlighted` icon name suffix if needed and loading the image.

I took extra care to fine-tune the colors to the best of my knowledge to make the icons easily recognisable, while providing a common look and feel.

As requested in #182 I've added a revamped version of the detail and tree view icons, and took the liberty of updating the add remote, fetch, pull and push icons.

As a little extra folder items also get a custom icon whether they are expanded or collapsed.

All icons should be available for retina (see attached screenshot below) and non-retina displays, if not let me know and I'll correct that.

Hope everyone likes the new look.

![screen shot 2013-06-09 at 12 50 42](https://f.cloud.github.com/assets/16507/628774/8a8f7816-d0f2-11e2-8557-3287f316f99c.png)