Better microtime support

[dakujem]

Improve support for times with resolution heigher than one second.

Ideas:

• double format parsing/formatting (?)
• implement: getSecondFraction
• implement: getMilliseconds, getMicroseconds
• implement: abs, ceil, floor, round ( to seconds / hours / days ... ) $this->_set(floor($this->toWeeks()) * self::WEEK);

[dakujem]

partially addressed in 4215b3a (implements Time::getSecondFraction)