Use sass version of bootstrap

[bhaettasch]

This allows easier customization

• Add sass compiler dependency and activate precompiling
• Use scss versions instead of css for bootstrap core and bootswatch theme
• Currently, all bootstrap components are loaded, but one might remove unused ones for further compression
• Set default font globally
• This also removes the unnecessary Google Font lookup
• Use local version of jquery
• Make sure all css and scripts are compressed
• Move first custom styles into theme variables