Grunt 0.4.0 files API & Async processing

Using https://github.com/treasonx/grunt-markdown/pull/7 and the comments from @treasonx I have added Grunt 0.4.0 file suport.

My new pull request contains all the work from @mattd
But it removed the options markdownExtension and htmlExtension
`markdownExtension` can be specified in the `src` and `htmlExtension` can be specified in the files array too.

The grunt file has been updated so the the markdown task works with the new files format.

Also I've added async processing, which from my tests is around 4x faster.

Before
![Screen Shot 2013-04-29 at 11 14 42](https://f.cloud.github.com/assets/678372/438046/6071fb3a-b0b8-11e2-9f7c-f851b6286814.png)
After
![Screen Shot 2013-04-29 at 11 22 15](https://f.cloud.github.com/assets/678372/438047/608e9f56-b0b8-11e2-93f1-67f8ed0e004a.png)

The version number has been upped to version 0.3.0 as this update would be a breaking change. And @mattd and myself have been added to the list of contributors in the package.json

Everything seems good from from what I can see.
What do you guys think?