Improve document type declaration processing

[opottone]

4 bugfixes and one improvement related to document type declarations:

• Properly escape system literal which contains a quote character "
• Properly serialize HTML declarations
• In elementsoup, fix processing of everything outside the root element (document type declaration, comments, processing instructions)
• Set/remove document type via lxml API
• Properly serialize comments & processing instructions which precede document type declaration

[scoder]

Looks good to me, thanks!