Finish MessageFormat implementation

[NightOwl888]

The MessageFormat class is only partially implemented. It was actually only ported for the use of ChoiceFormat, which is required by Transliterator to load resources. But it currently doesn't work much beyond that purpose.

MessageFormat has many dependencies, including DateFormat and RuleBasedNumberFormat that would also need to be ported in order to make it complete.

Ideally, these format classes should implement ICustomFormatter and IFormatProvider to be compatible with string.Format() and other .NET APIs. Also, they should ideally not utilize CultureInfo as a property themselves, but be passed a CultureInfo instance when they do their work. That might not be feasible in all cases (such as rule-based number format). More analysis is required to work out the best approach for .NET compatibility.

[NightOwl888]

For now, this class has been marked internal so it can be redesigned and dealt with at some later point.