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?