Merge pull request #6 from jcoleman/update-prawn-version-dependency

Allow prawn in the 1.0+ version range.
Add change log
Minor tweaks to the change log
Make change log say upcoming
Fix extension build so that gem installation uses the same path as the rake test.
Currently when you attempt to load the installed gem, you get the following error:

    /lib/prawn/gmagick.rb:2:in `require_relative': cannot load such file -- ...bundler/gems/prawn-gmagick-181cfeb75911/lib/prawn/gmagick/image (LoadError)

because the extension compiles into lib/image.bundle rather than lib/prawn/gmagick/image.bundle like the Rake task triggered under testing used. This patch standardizes on the default gem installation behavior.
Don't have a runtime dependency on rake-compiler.