Add __tostring metamethods to prototypes

This is nice to have, and aids with debugging and interactive usage.

matrix/api.lua

@@ -27,6 +27,7 @@ end
 
 
 local API = {}
+API.__name  = "matrix.api"
 API.__index = API
 
 setmetatable(API, { __call = function (self, base_url, token, http_factory)
@@ -40,6 +41,10 @@ setmetatable(API, { __call = function (self, base_url, token, http_factory)
    }, API)
 end })
 
+function API:__tostring()
+   return self.__name .. "{" .. self.base_url .. "}"
+end
+
 function API:_quote(string)
    return self._http:quote(string)
 end