Merge branch 'master' into yummin
Change process_execute() in process.c

1. Modify README
2. Add DESIGNDOC for Project 2
Implement some functions in process.c, syscallc.c
Simplify the implementation of process_execute
Implement functions in process.c

Implement process_wait and process_exit, rewrite start_process.
Passed some tests.
Update code: Implement some functions
Fix typo and add functions
Update the code in userprog
1. Implement create() and remove();
2. Modify process.c
Merging from branches/borui to branches/yummin
Cleanup code
Update repository
Merge branch 'yummin'