Abstract digest class

When using compass to compile sprites, we have run into an issue where Digest::MD5 is not an acceptable digest algorithm -- specifically for (Linux) hosts running in FIPS-enabled mode.  As such, this proposed change is what we're using to abstract the digest class used.  The default is Digest::MD5, but in our application we override it with Digest::SHA2 in our compass.config.

I'm not sure what tests I can/should create.