Rename ProductSpace to TupleSpace

[jonashen]

OpenAI gym's naming convention is to call a product space TupleSpace. Thus, this PR renames all instances of ProductSpace to TupleSpace to keep garage consistent with gym.

Ref: #287

[codecov]

Codecov Report

Merging #338 into master will increase coverage by 0.04%.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##           master    #338      +/-   ##
=========================================
+ Coverage   62.06%   62.1%   +0.04%     
=========================================
  Files         213     213              
  Lines       14349   14349              
=========================================
+ Hits         8906    8912       +6     
+ Misses       5443    5437       -6

Impacted Files	Coverage Δ
garage/theano/spaces/tuple.py	80% <100%> (ø)
garage/theano/spaces/__init__.py	100% <100%> (ø)	⬆️
garage/tf/spaces/__init__.py	100% <100%> (ø)	⬆️
garage/theano/envs/base.py	90.9% <100%> (ø)	⬆️
garage/tf/spaces/tuple.py	71.42% <100%> (ø)
garage/tf/spaces/dict.py	81.81% <50%> (ø)	⬆️
garage/tf/envs/base.py	73.58% <50%> (ø)	⬆️
garage/theano/spaces/dict.py	63.46% <50%> (ø)	⬆️
.../theano/optimizers/conjugate_gradient_optimizer.py	72.15% <0%> (-1.9%)	⬇️
garage/envs/mujoco/gather/gather_env.py	56.56% <0%> (+0.33%)	⬆️
... and 2 more

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[coveralls]

Coverage decreased (-0.01%) to 62.629% when pulling 6fc66be on product_to_tuple into f798f1e on master.