RGB <-> LabSpace memoization

PR started to address issue #9 . This is work in progress for comments and direction.

** Note: This code won't yet compile.
Instead of memorizing them here, what about memoizing these at the calling site (i.e. inside `DominantColors.swift`)? 

With this implementation, the conversion functions are no longer pure because they share global mutable state in the form of the memoization dictionary. This may have unintended consequences, as the cached values will stick around for the entire lifetime of the application.