[Exist-open] Function round-half-to-even

[Jens <je...@fa...>]

Due to the fact that I needed the missing standard xquery function 
round-half-to-even to round values to a given precision, I have tried to 
implement it on my own. 
It is my first contribution to the eXist-project, hopefully I have done 
everything in an accurate way. If not, feel free to correct me... ;-)

Anyway, in order to get the function running, I have chosen to add a 
round(IntegerValue precision)-method to the class NumericValue and its 
implementing classes DecimalValue, DoubleValue, FloatValue and IntegerValue. 

Of course, the attached patch includes the new function module called 
FunRoundHalfToEven. I have also added a new test case called 
XQueryFunctionsTest, which tests the new xquery function with some tricky values 
(mainly the examples from W3C). Maybe it would be wise to have tests in there 
for other xquery functions as well (I have not found many so far)?

Jens