Better handling of headers and sub-contents in KMime::Content: * a factory for header types * greatly simplify Message and NewsArticle (headers treated in a generic way) * proper distinction between string-representation and broken-down-representation, with proper docs for parse() and assemble() * provide a 'frozen' property for content that we want to keep untouched

Reviewed at http://reviewboard.kde.org/r/1151/ .

svn path=/branches/work/akonadi-ports/kdepimlibs/; revision=1006180