allow some deprecation warnings to be silenced (take 2)

Previously, a deprecation warning was issued instructing the user to
load the core extensions explicitly. This warning continued to be
issued even if the user added code to require the core extensions (as
instructed). This change sets a class variable on Money when the core
extensions are loaded appropriately so that the user isn't annoyed by
unnecessary deprecation warnings for the functionality they're no longer
making use of.

*Note:* This pull request obsoletes a previous attempt (#335)