C++ Network Library Code

Refactored message_test.cpp and url_test.cpp.
Replace references to std::string with string<Tag>::type, but this is something that is tightly coupled to the HTTP parser, so more work needs to be done to fix this.
All tests are successful under gcc on Linux.

Cleaned up traits subdirectories.

Refactored traits from message so it might be easier to reuse them.

Refactored traits and tags so that they are not duplicated for message and URI.

Merged 71-HEAD from branches/urllib-dean

Updated some more docs. They're not complete and still need reviewed.

Supporting +,-, and . in the scheme part as per RFC 1738.

Adding uri example.

Implementing basic_uri constructors that take a string as a parameter.

minor cleanup