Feature/remove term migrate

Two commits to clean up.

b774fd9 is of #18. This patch renames all WBXXX macros to DMXXX (e.g. WBERR -> DMERR).

b3454dd is of http://www.redhat.com/archives/dm-devel/2014-July/msg00059.html. Unfortunately, the proposer Satoru couldn't make time for writeboost and this is why I submit a patch.

Big patch though, what's done is simple and easy.