Configurable table prefix

[kPshi]

Allows to set a TableMapper (similar to the FieldMapper) field in DB which then may return different table names even within the same process for different DB configurations.
The GetTableName function has been renamed in order to match the naming of DefaultFieldMapFunc and in order to prevent hard to find bugs later on.

Usages of GetTableName should ideally rewritten to use

DB.TableMapper

For setting a prefix per instance of DB use

db := NewFromDB(myPlainDB, "mysql")
db.TableMapper = func(a interface{}) string {
    return "myprefix_" + DefaultTableMapFunc(a)
}

[coveralls]

Coverage increased (+0.02%) to 89.188% when pulling 33fd39f on kPshi:configurable_table_prefix into b7add2e on go-ozzo:master.

[qiangxue]

Thank you for your contribution! I've merged your code manually.