Fixing non-default navbar style in mean-admin module

Issue #21 on the mean-admin repository (https://github.com/linnovate/mean-admin/issues/21) discussed how the use of the `navbar-inverse` class on the main navbar was causing all of the styles applied from http://bootswatch.com/ in the themes feature to show different than advertised because they all applied the *inverse* navbar style. @fyockm suggested that I correct the base css to allow the `navbar-default` style to be used while still maintaining the current black navbar.

I've done so here, and I've also added `less` to the grunt workflow. That may seem like the sledgehammer approach to getting this done, but the css for overriding the standard bootstrap navbar was really gnarly without using a css preprocessor to get it done. I hope that's OK... I've wanted to use `less` in my MEAN.io projects anyway.

With this change the everything should still look exactly the same, but now the current black navbar look is pulled in through the `navbar-default` class while the bootstrap standard white navbar has been set as the `navbar-inverse` style.