Implement attachments parameter for GetDocument method in
CouchDatabase.cs. This allows you to fetch a document and all its
attachments in one request, by adding an ?attachments=true URL parameter
to the GET request. Although asking for a MIME multipart/related is more
efficient, it can sometimes be useful to just do it in one request.

Read
http://wiki.apache.org/couchdb/HTTP_Document_API#Getting_Attachments_With_a_Document
for more information about this subject.
Improve readability of CouchDatabase.cs by using string.Format() where applicable.
Merge pull request #44 from jvdgeest/master

Implemented attachment parameter for the GetDocument methods and improved readability a bit for CouchDatabase.cs