Custom obtvse overrides for default Gist styling

## Custom obtvse overrides for Gist styling

### The issue

* Default embedded gist line numbers do not support word wrapping and hence break when wrapping is enforced in pages/divs with limited width: they no longer match the lines and generally are crudely styled
* Embedded gists inherit some **obtvse** styling better suited for posts/content rather then code

### The solution

This commit adds a few css rules to achieve the following:

* hide the default gist line numbers and implement css-counter
  based line numbering, accommodating word wraps and perfect fit between
  numbers and lines
* line-height and font-size modified for better presentation of code;
  previously they inherited obtvse styling for post text and presentation

### A picture is worth a thousand words

#### Picture Before
![before](https://f.cloud.github.com/assets/285772/94020/f3e39574-663d-11e2-8055-6718cf1e2a07.png)

#### Picture After
![after](https://f.cloud.github.com/assets/285772/94021/fa4e62ae-663d-11e2-9957-52479cdc622c.png)