Add multi-row header support
Fix additional table rows height logic
Fix failing tests
First attempt at specs for multiple-row headers
Removed non-existing, referenced files from Rakefile so that 'rake rdoc' works again
Merge pull request #564 from gettalong/rakefile-fixes

Removed non-existing, referenced files from Rakefile so that 'rake rdoc' works again
Merge pull request #566 from weimeng/multi-header

Add multi-row header support for tables
Cell#style ignores irrelevant attributes

This allows overbroad :cell_style arguments to be passed in, which are
only applied to cells that recognize them. Previously, the only way to
do this would be to selectively style only certain kinds of cells based
on their types.

Closes #515.
Add better exception messages with MalformedPDFError
Merge pull request #567 from cosmic/master

Add better exception messages with MalformedPDFError
Refactor so images via Pathname are read in binmode

* Separate out image IO opening and verification logic
* test that embedded images parse correctly (this was the problem that
  was being masked before)
* Add simple matcher to tidy up checking XObject parseability
* fixes prawnpdf/prawn#570:

Pathname instances respond to #read but not #binmode, and the old code
was assuming that anything which responded to #read was an IO, with the
result that Pathname instances were beign treated as IO's and getting
read called on them, bypassing File.binread (things responding
to #binmode have that called so with a real IO it would have #read
called after it had been put into binmode...

The fix is simple enough: treat everything that responds to binmode as
an IO (IO's and Files), and anything else treat as a path, and open it
'rb'.