Fix indentation handling

Indentation problems were mostly fixed by 7976bc. However, there were some cases where `tab-width` could be modified after mode initialization (manually by the user, by local file variables, by some minor mode). The first commit fixes that by using `tab-width` instead of `coffee-tab-width` everywhere, except at places were we set default values.

Moreover, the default indentation for Coffee scripts should be 2 as many projects have agreed to use that as a default value. This seems a more sensible value than 8 (the default `tab-width` value).