[BUG] Default values of scale and precision are reversed.

Resolves error where the precision value must be greater than the scale
value in MySQL. Have also updated PostgreSQL adaptor for consistency.

Detail of the error can be found at
http://dev.mysql.com/doc/refman/5.0/en/error-messages-server.html#error_
er_m_bigger_than_d
Merge pull request #84 from sweetbee/bugprecisionscale

[BUG] Default values of scale and precision are reversed.