Bug and crash fix in fast_compile

This fix bugs on the CPU in fast_compile introduced in:
commit 4f8f0da5
Date:   Thu Feb 27 09:56:12 2014 -0500

Add tile in the doc.

Update the c_code_cache_version that was forgotten.

Mixed

Speed up matVecModM function.

Add consider_constant op

Use pip --user instead of --install-option

I don't know if there is a specific reason that --install-option='--prefix=~/.local' is used instead of pip's --user, but it seems to break things when installing Theano without root access (both myself and Jae Hyun had this issue).

Moved consider_constant from theano.tensor.extra_ops to theano.gradient, moved the optimization that removes it to theano.tensor.opt. Modified theano.compile.builders so it doesn't import theano.gradient, because theano.gradient now imports ViewOp from theano.compile and this would lead to a circular dependency.

Add new subtensor helper make_constant and use it to fix a bug in local_subtensor_merge.

Fix gpu crash and faster optimization

Add fast_compile tag so the optimization to remove the Op always runs. Use g_out.zeros_like instead of tensor.zeros_like so the op also works for sparse variables.

We take care to still have this order: gpu, cpu/gpu fusion, then gpu again.

fix crash when we selected the wrong version of the gpu conv code.