ng-grid version 2.0 - Beta

opening pull request for comments as well, we are still adding a few things before calling it done.

This potentially can break certain templates already in place. we had to make the change to default templates and usage of them to accommodate the grid virtualization (especially heavy virtualization mode)

Additions:
1. Column virtualization - modes normal and 'heavy'
A heavy virtualization turns off ability to pin columns or show the selection checkbox and all templates must be uniform, but performs about 25% faster while scrolling than the built in virtualization.

2. Multi-column sorting
Hold shift and click on the header text to multi-colmn sort. When multi-sorting, A small number indicator appears in the top left corner of the header to indicate sort priority.

3. Excel-like editing
Can now start typing when on focused cell and will start editing cell. Can also double click cell to start editing cell. 

Changes: 
1. Columns are always virtualized. 
due to some architectural changes, some templates may no longer work. The wiki templating guide will be updated with the release of 2.0.
2. Editable template changes.
We are no longer using ng-model for the default editable input template. It is a custom directive that is smaller and specific to the grid. You will be able to specify your own templates, this is just FYI.
3.Removed edit on focus due to it not playing well with virtualization.

fixes:
1. memory leak present in 1.8+ in chrome.
2. scrolling is now smooth instead of choppy, especially in grids with a larger view-port area.
3. Many many many performance related issues.