Gpu max

This fix a new test for a new feature.

If travis would have run the tests, it would have detected it, but it didn't ran...

This fix a crash recently added in 5178e4

added eval method and tests

Gpu alloc2

We need to update the second var before we use it!

This is a significant speed up with the gc as most of the time, we
don't need it and allocating on the GPU is slow.

C code gpusub

Also add missing decref in error handling code.

Revert "Optize GpuSubtensor by 11% by moving the reversed call in the make node instead of the perform."

This reverts commit aeedaa73.

We will reuse the cpu Subtensor c_code, so we need the parameter to be
in the same order.

So we use PyArray_BYTES instead of PyArray_DATA

-use range instead of a tensor with the same values
-small refactoring to make the code easier to read.