Strip !important from property values.

Before gathering stats, preprocess all declaration values containing
!important in them, stripping the "!important" string from the end
and annotating the declaration object with important:true property
to not lose the information of being important in case this will
be needed for some future processing.

Created iterator module to easily iterate over all declarations -
copy-paste code from lib/declarations.js - needs to be refactored.
Re-use the iterator in declarations module.

Removes duplication introduced earlier.