Add a feature of loading templates from binary data generated by go-bindata

I added a feature of loading templates from binary data generated by go-bindata.

I created [`martini-contrib/staticbin`](https://github.com/martini-contrib/staticbin) so that Martini can serve static files from binary data.

By adding this feature on `martini-contrib/render`, a web application using Martini can be compiled into a single binary file completely.

I would be glad if you could merge this PR.

[Changes]
* Added the fields of `Asset`, `AssetName` on `Options`.
* Cut out the inner logic of `filepath.Walk` to `load` function.
* If `options.Asset` and `options.AssetNames` are set, load templates from binary data. Otherwise, load templates from files as before.

I thought of creating a new middleware which has this feature. But `martini-contrib/render` had various important features and I thought it was not good idea to create a new middleware similar to this one, so I decided to add this feature on the existing  `martini-contrib/render` middleware.

Thanks.