Fix of HEX-1571: duplicate name in frame during scoring.

Score code attached new columns which names correspond to names of
response classes.
If dataset contained no column names (1,2,3,... were assigned) and
response column contained
numeric classes (0,1,2 for example). Then score failed on adding a new
column with duplicate name.

Now, the columns are prefixed by "class_" prefix if there is a
duplicity..
Merge branch 'master' of github.com:0xdata/h2o