Don't escape backslash before underscore or percent

Because of the way LIKE clause right-hand-sides are escaped, a
backslash before a percent or underscore doesn't need to be escaped in
a string.
LIKE expressions
Respond to diff comments: mostly more doc and test
Merge pull request #83 from sixolet/master

LIKE expressions for sqlbuilder
Update contributor list
Properly cache empty values
Make value validation return an error
Include value length in error message
Merge pull request #84 from adegtiar/fix_memcache_empty_value

Properly cache empty values