NEXUS-7521: simple Asset hashing service

based on Guava's thread-safe hashing functions

https://issues.sonatype.org/browse/NEXUS-7521
http://bamboo.s/browse/NX-OSSF345-1 :white_check_mark: 
Correct, so the question is then do we expect plugins to come along and contribute non-core hashing algorithms that will be used by the core? Since there's nothing stopping a plugin from using its own hashing algorithm in place of this core set of known hashing algorithms (ie. by consuming the asset stream directly and spitting out a hash).