Drop TODO talking about adding 64bit support --install-wkhtmltopdf.

The install-wkhtmltopdf command has been dropped. It therefore doesn't make sense to keep this TODO.
Support :except option in middleware.

The :except option in the middleware specification accepts the same kind
of parameters as the :only option. A possible usecase is to use direct
PDF rendering using prawn or another library for specific views but use
PDFKit for all other pages.
Added rake to the gem file for Travis-ci builds.
Worked now lets put the rake build requirment in with the others.
Note about proper HTTP response headers
Update link now that PDFKit has its own org
fixed a link to wiki in POST_INSTALL message
test on travis, requires adding wkhtmltodpf-binary as a dev dependency
Typo
Incorporating wkhtmltoimage
Adding image format shortcuts
Updating readme to reference wkhtmltoimage
Fix installers comment
Bump version for 0.5.2.galileo.1
Merge branch 'master' of github.com:barbolo/PDFKit

Conflicts:
	README.md