Added Grunt.js integration for compiling less files and TravisCI for testing the build

Hello,
I had the issue (#76) of publishing bootswatch to the NuGet Gallery, but I had to manually edit the image paths for every single `variable.less` file, since the default images path in .NET Web applications is `Content/images/`; and I probably had to do it every time you pushed something new.

So I introduced a `global` folder which contains:
- `build.less` and `build-responsive.less`, templates used during file generation.
- `global.less`, a file that holds global variables that, if set, override the ones in the single `variable.less` of each theme.

### How to build
Building is pretty straightforward:
At first, make sure you have `node` and `npm` installed. Then:

```bash
$ npm install -g grunt-cli recess bower
$ cd ~/path/to/bootswatch
$ bower install bootstrap
$ grunt swatch:name_of_swatch
```

If you don't specify a swatch name (`grunt swatch`), all of the themes will be built.
Built themes are put into the `build/` folder in the root.

Also, I have added the `bootstrap` dependency to the bower package, so that jQuery is handled as a nested dependency.

I have also integrated TravisCI in my fork, in order to test the building system for every push I make.

You can avoid the deletion of the `CNAME` file, it caused me troubles when committing and pushing to my forked repo.