Merge branch 'yummin'
Modify thread, thread_create() and thread_set_priority()
Merge pull request #1 from yumminhuang/borui

Modify thread, thread_create() and thread_set_priority()
Merge branch 'master' into yummin
Modify codes in src/thread

1. Add comments and DESIGNDOC
2. Rename member in thread struct
3. Reformat code