#5 Strip encoding information from compiled CSS

By default, when hsass outputs UTF-8 characters outside of the ASCII range, it prefixes its output with encoding information. Depending on the level of compression, this is either in the form of @charset "UTF-8" or a Byte-Order Mark. This is problematic, since Yesod does not account for the BOM when concatenating CSS. As a result, the BOM ends up in the middle of concatenated CSS (which was causing a divergence between development-mode and non-development-mode CSS on Snowdrift).

I had to make a few changes after testing this against Snowdrift. It seems that importer now needs to return absolute paths (probably because we're using a later version of libsass).

Unfortunately this only partially fixes the production/development CSS disparity. For some reason, fonts.silius isn't being compiled correctly. I wonder if Shakespeare is struggling to parse the compressed CSS as Lucius.