Install generator improvements

- Adds support for haml (refs ahuth/emcee#18)
- Modified the generator script to be more intelligent in selecting the default layout file to modify
- Modified the generator script to give the developer the option to specify a path for the layout.
- Modified the way content is injected so that it preserves the white space and indentation (especially important for haml and slim )
- Incorporated feedback from @ahuth for layout_file method