CM: handling mixed domains in more user-friendly way.
PUB-1637: more user-friendly sorting for mixed domains.

During CM construction actual vector and predicted vector
domains are merged together by a set union.
The set union is clever in this way:
 - if both domains contain only string values, String#compareTo is used
   (preserve lexicographical ordering)
 - if both domains contain only numeric values, numeric order is
   preserved
 - if domains contain mixed values, then numeric values are sorted in
   numeric order.

Comment: all these operations should be probably performed only by UI
layer, on the other hand it is handy to preserve numeric ordering
in CM domains also during computation since we need to print
temporary results into logs.
Merge branch 'master' of github.com:0xdata/h2o