Support text attachments in HTML Writer

if the NSTextAttachment conforms to <DTTextAttachmentHTMLPersistence>

First baby step in implementing #611.
Allow building under iOS 6

although I don't know whoever wants this ;-)
Fixed previous commit
Merge pull request #730 from futuretap/attachments

Support text attachments in HTML Writer
Fixes build with Xcode 5.1
Refreshed DTFoundation link
Set DTFoundation to 1.6.2
removed unused code #735
Removed manual underline drawing on iOS 7, fixes #723
Merge branch 'od/issue_723' into develop
Fixed DTFoundation docs
Reverted pull #698, because contained 7d60edcb1af4 is the cause for #731
Merge branch 'od/issue_731' into develop
Added unit test for #740
prevent trimming of HR field inside of block, fixes #740
Made check more precise. In 64-bit unit test I also saw NSIntegerMax as a result here.
Merge branch 'od/issue_740' into develop
Added screenshot button to demo, #727
Added content scale sanity check #727
Merge branch 'od/issue_727' into develop