Fix case of all-zero response class

Happens if GBM gets a perfect prediction & more trees keep on coming.
Also, turn on GBMTest in the JUnits
Merge branch 'master' of github.com:0xdata/h2o
Turn off key-leaking GBMDomainTest

Real issue the new mapping Vecs are not being collected nicely.
Make cleaner versions of Vec.remove also.
Covering case when a test column contains more enum values
than the corresponding column in test data.
Proper deletion of adapted frame.

The deletion is tricky - the code records all newly created
vectors into a dedicated frame and delete it.
Dummy fix of columns field description.

This is not conceptual change since it propagates help string
only for one particular UI element.
Fix of source input field bug.

The source input field was missing a check for required flag.
So any name of key was able to be passed as a valid key even it did not
exist.
Merge branch 'master' into grids