Microseconds

first attempt to add full support for microsecond resolution, comes with a major improvement of the tests for TIME, DATE and DATETIME.

At the very least, this can be built upon.
For a time when I deeply mistrusted the decimals field, I did use it.
I think we should still keep it for two reasons:
* it provides a hint useable for the maximal allocation size in all types
* with it, mysqlField is perfectly aligned on 64bit boundaries