Add round

[oxinabox]

This adds rounding.

Including round and cast, eg round(Int, x),

For interest/trackback: http://white.ucc.asn.au/2017/05/04/Plain-Functions-that-Just-Work-with-TensorFlow.jl.html

[codecov-io]

Codecov Report

Merging #213 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   62.92%   62.93%   +0.01%     
==========================================
  Files          48       48              
  Lines        3361     3362       +1     
==========================================
+ Hits         2115     2116       +1     
  Misses       1246     1246

Impacted Files	Coverage Δ
src/shape_inference.jl	83.33% <ø> (ø)	⬆️
src/ops/math.jl	86.27% <100%> (+0.06%)	⬆️

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