implment task 2.2, non-tested
Implemented LRU replacement

Signed-off-by: tianyuli <tli@wpi.edu>
NPE revised version

Signed-off-by: tianyuli <tli@wpi.edu>
merge from jzhu_master_copy except BasicBufferMgr.java
only 4 empty buffers
Empty buffer test done
test empty buffer version 2.0 done
add delete initialized unnecessary db files
test task 2.2 almost done
start test task 2.3 LRU
test task 2.3 LRU done
add some testing description
delete unnecessary report.docx
add file comments
rename simpleDB's readme.txt
Final Version

Signed-off-by: tianyuli <tli@wpi.edu>
add documentations - readme.txt and testing.txt