Refactor http::header contents (API Change):

fix #124

The http::header data members "method", "url", and "reason"
are changed from data members, to pairs of get and set
functions which forward the call to the Fields type used
to instantiate the template.

Previously, these data members were implemented using
std::string. With this change, the implementation of the
Fields type used to instantiate the template is now in
control of the representation of those values. This permits
custom memory allocation strategies including uniform use of
the Allocator type already provided to beast::http::basic_fields.

doc/examples.qbk

@@ -35,8 +35,8 @@ int main()
 
     // Send HTTP request using beast
     beast::http::request<beast::http::string_body> req;
-    req.method = "GET";
-    req.url = "/";
+    req.method("GET");
+    req.target("/");
     req.version = 11;
     req.fields.replace("Host", host + ":" +
         boost::lexical_cast<std::string>(sock.remote_endpoint().port()));

doc/http.qbk

@@ -113,7 +113,7 @@ that type.
 This illustration shows the declarations and members of the __header__ and
 __message__ class templates, as well as the inheritance relationship:
 
-[$images/message.png [width 650px] [height 390px]]
+[$images/message.png [width 711px] [height 424px]]
 
 For notational convenience, these template type aliases are provided which
 supply typical choices for the [*`Fields`] type:
@@ -137,8 +137,8 @@ object:
     ```
     request<string_body> req;
     req.version = 11;   // HTTP/1.1
-    req.method = "GET";
-    req.url = "/index.htm"
+    req.method("GET");
+    req.target("/index.htm");
     req.fields.insert("Accept", "text/html");
     req.fields.insert("Connection", "keep-alive");
     req.fields.insert("User-Agent", "Beast");
@@ -148,7 +148,7 @@ object:
     response<string_body> res;
     res.version = 11;   // HTTP/1.1
     res.status = 200;
-    res.reason = "OK";
+    res.reason("OK");
     res.fields.insert("Server", "Beast");
     res.fields.insert("Content-Length", 4);
     res.body = "****";
@@ -295,8 +295,8 @@ operations performed). To send messages synchronously, use one of the
     {
         request<string_body> req;
         req.version = 11;
-        req.method = "GET";
-        req.url = "/index.html";
+        req.method("GET");
+        req.target("/index.html");
         ...
         write(sock, req); // Throws exception on error
         ...