Updated things to make it work now, and included bootswatch themes

This repo wasn't updated for 2 years. Today i tried to implement some bootswatch themes to sonata admin, but this repo was out of sync with SonataAdminBundle.

Problems:

1. Current version of less doesn't work with it.
2. It modifies `less` npm module to achieve custom css class.
3. It still uses 2.0 version of bootstrap, whereas SonataAdminBundle uses `v2.1.0`

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My Solve: (included in PR)

1. Version `1.3.3` is required to build the less files. So i included `1.3.3` version of less with it.
2. I patched the `ruleset.js` file of included `less`. I also provided a patch file so users can patch it in future.
3. I linked the `v2.1.0` of `bootstrap` in git submodule.

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Addition: (included in PR)
- Added bootswatch as submodule and a Makefile to build bootswatch themes for SonataAdminBundle.