Added forward_features inside Estimator class of Adanet.

[chamorajg]

Added forward_features function inside estimator class to use tensorflow's forward_features inside adanet. Closes #114 @cweill

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 0.11%. The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   88.94%   88.82%   -0.12%     
==========================================
  Files          31       31              
  Lines        3011     3016       +5     
  Branches      475      476       +1     
==========================================
+ Hits         2678     2679       +1     
- Misses        244      248       +4     
  Partials       89       89

Impacted Files	Coverage Δ
adanet/core/estimator.py	90.68% <20%> (-0.62%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 88daf39...f03fe72. Read the comment docs.