Refactoring source files and directories; Some documents.

This commit try to make sure we can writting install script for most OSs easily.

SNMP Agent and SNMP library are seperated to different directories.

Changes:

- SmartSNMP libraries:
  - All lua library move to `lualib/smartsnmp`, so we can *require* directly.
  - SmartSNMP lua C module will be built to `build/smartsnmp/core.so`.
- SmartSNMP agent application:
  - Move SNMP Agent `snmpd.lua` to `bin/snmpd`, later it will be install to `/usr/sbin/` or some where.
- SmartSNMP agent MIB node implementation.
  - Move `example` to `mib`.
  - In future, *private* mib should be placed to `private` directory.
- Remove lots of global object.
- Import getopt lua function for parsing arguments in `snmpd`.
- Simple document in doc/api.md.

Signed-off-by: Xiongfei(Alex) Guo <xfguo@credosemi.com>