Try to make SmartSNMP can install in most OS easily; Import config file.

This PR try to make SmartSNMP can install in most OS easily, and it start to use a configuration file which include the SNMP agent config and where to find the lua MIB module.

- Refactoring source files and directories;
- Some documents.
- Configration file load; 
- Load MIB module files specified in config file.