CONPY-49: Added support for Decimal type

1) When retrieving data with column type MYSQL_TYPE_NEWDECIMAL C/Python
now loads the decimal module and converts data from string into Pythons
decimal.Decimal type.

2) Wnen sending a decimal.Decimal parameter, value will be converted to string
and send with type MYSQL_TYPE_NEWDECIMAL to server.

include/mariadb_python.h

@@ -273,6 +273,8 @@ extern PyObject *Mariadb_NotSupportedError;
 extern PyObject *Mariadb_Warning;
 
 extern PyObject *Mrdb_Pickle;
+extern PyObject *decimal_module,
+                *decimal_type;
 
 /* Object types */
 extern PyTypeObject MrdbPool_Type;