Adding SQL file for quick update to VARDIFF

This branch now includes all recent changes from `next`! It might have
broken some things so @obigal needs to test it again, but at least we
can now continue on this with all recent additions added to this branch.

Addresses #145

sql/issue_145_shares.sql

@@ -0,0 +1,2 @@
+ALTER TABLE  `shares` ADD  `difficulty` FLOAT NOT NULL AFTER  `solution` ;
+ALTER TABLE  `pool_worker` ADD  `difficulty` FLOAT NOT NULL AFTER  `password` ;