bump pdf-reader dependency to 0.12.0.alpha

* provides a much nicer API for loading data from existing PDFs
add support for embedding existing PDF files

* handy for loading vector images
simplify importing of pages from an existing PDF

* rely on a PDF::Reader::Page object to represent a single page from an
  existing file and to gather all the appropriate attributes for the
  page
ObjectStore#load_object_graph shouldn't modify Hash objects in-place

* should create a new hash
ensure we get the root PDF module
options should be optional here
add option to ignore certain attributes when importing a page

* the page importing includes all inherited attributes, so we never
  want to import the Parent link
* when importing a PDF as a Form XObject we want to skip the contents.
  Content will be imported in a separate stage
update documentation to mention PDF image support
bump pdf-reader dependency to the latest beta gem
cells proxy should only pass on methods that each cell recognises
use floating point maths to avoid truncating to 0
use a stable release of pdf-reader
relax pdf-reader dependency slightly

* any 1.x series will work fine
Merge branch 'master' into pdf_images

Conflicts:
	Gemfile
	lib/prawn/core/object_store.rb
	lib/prawn/images.rb
	lib/prawn/table/cells.rb
	prawn.gemspec
Merge remote-tracking branch 'origin/pdf_images'