add cache for string
cache for string primary key
cache support all kinds of primary keys
Fixes #179
bug fixed
Merge branch 'master' of github.com:go-xorm/xorm
Merge branch 'bugfix-deleted-find' of https://github.com/oinume/xorm into oinume-bugfix-deleted-find

Conflicts:
	session.go
Merge branch 'oinume-bugfix-deleted-find'