The lock around instantiating custom types is really expensive

Don't take that code-path if there are no registered custom types.
Merge remote-tracking branch 'jstedfast/master' into fluid-ctors
All message parts types except crypto ones have fluid ctors
Code cleanup
Support for content (Stream and IContentObject) in MimePart fluid ctor

Note that to support IContentObject and not just ContentObject this
changeset also modifies the signature of the ContentObject property.
More constructor tests
MonoDevelop specific project