Add cuda compute capabilities 3.0, 3.5, 5.0

Untested.
Update in response to discussion. Still untested and unsuitable for merge.
Merge branch 'master' into cuda_30_35_50
Output cuda device attrs when in debug mode
Turn simplify_exprs back into simplify-except-for-lets

fixes cuda test failure
Emit the correct PTX ISA
cuda compute capability 5 uses libdevice 2.0
Fix missing ampersand
Add device attribute enum to mini_cuda and use it
Error on ptx isa 4.0 for llvm < 3.5
Merge pull request #375 from halide/cuda_30_35_50

Add cuda compute capabilities 3.0, 3.5, 5.0