yzliao_v0.2_issue_21

This pull request address #21. Here is a list of changes :

1. replace SN_ID with node_id, which is a global variable shared by different files
2. initialize slot_number as node_id
3. improve the way to write/read node_id and now it is capable with Cooja simulator right now
4. modify the way to set AP rime address and allow the AP to have different node ids (other than 0)