Improve XMPPDateTimeProfiles to allow any number of fractional digits.

According XEP-0082, section 2.1, "sss (...) (MAY contain any number of
digits)". Allow any number of fractional digits in the parser (at least one,
if the decimal separator is there).

Add tests to show both 3 and 6 fractional digits are ok.

Caveat: NSDate precision is only three decimal digits, so parsing the string
into a NSDate loses precision. Anyway, that's still better than providing you
a nil value.

BTW, is there any interest in a proper (meaning OCUnit or GHUnit) test solution?