Accept a block for attr_initialize and friends

The block is useful when using attr_extras in conjunction with classes
like SimpleDelegator from Ruby's standard library. It's also helpful for
setting up memoized lookup hashes.
Move pattr_initialize, vattr_initialize specs
Add documentation about initializer blocks
Merge pull request #5 from jferris/jf-accept-initializer-block

Accept a block for attr_initialize and friends