test_backprop: remove cruft left over from old metadata format
recur-nn: bottom layer uses rectifier activation

It turns out it was plain linear.
recur-nn: Try rectified sqrt units

The activation is MAX(sqrt(x + 1) - 1, 0), and its derivative is
0.5 / sqrt(x + 1).  This allows the same zero-case short circuits as
ReLU, and is similar to ReLU at the hinge (though half scale).

Seems to work.
recur-nn: avoid division by zero if no error is back-propagated
Add activation options (ReLU, ReSQRT, ReLOG)

Only test_backprop has the options now.

1. ReLU is the old activation -- MAX(x, 0)

2. ReSQRT is MAX(sqrt(x + 1) - 1, 0)

3. ReLOG is MAX(log(x + 1), 0)

ReSQRT and ReLOG seem quite fast and stable.
recur-nn-helpers: fix soft_clip once and for all
test/text_exp: look closer at logf in the useful range