v0.3.4

Refactored and added some methods.

• orderBy() now you can use multiple fields for sorting (use an array like ['field1', 'field2 desc'])
• groupBy() now you can use multiple fields for grouping (use an array like ['field1', 'field2'])
• where() and having() now you can omit the operator = or IN, usage where([['id', 3]]) and where([['id', [10, 11, 12]]])
• method pluck($key_index, $column_index) returns an array of needed columns
• method exists() returns true if SQL query result has a row