yzliao_v0.2_issue_21

This pull request:

1. replaces SN_ID with node_id, which is a global variable shared by different files
2. initializes slot_number as node_id
3. improves the way to write/read node_id and is capable with Cooja simulator right now
4. modifies the way to set AP rime address and allows the AP to have different node ids (other than 0)