WhereClause: A new dedicated builder for WHERE

[huandu]

Due to the importance of the WHERE statement in SQL, we often need to continuously append conditions and even share some common WHERE conditions among different builders. Therefore, we abstract the WHERE statement into a WhereClause struct, which can be used to create reusable WHERE conditions.

SelectBuilder, UpdateBuilder and DeleteBuilder have a new anonymous field WhereClause now to expose necessary APIs. Read the samples in the file whereclause_test.go as HOW-TO documents for WhereClause in all kinds of scenarios.

[coveralls]

coverage: 96.32% (+0.02%) from 96.304%
when pulling 3771160 on feature-where-clause
into c35040f on master.