Use Optim instead of home made Newton-Raphson

[andreasnoack]

Fixes #11 and hopefully fixes #33

[codecov-commenter]

Codecov Report

Merging #34 (0fc6e0a) into master (9135253) will increase coverage by 11.60%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #34       +/-   ##
===========================================
+ Coverage   87.94%   99.54%   +11.60%     
===========================================
  Files           6        6               
  Lines         141      222       +81     
===========================================
+ Hits          124      221       +97     
+ Misses         17        1       -16     

Impacted Files	Coverage Δ
src/Survival.jl	100.00% <ø> (ø)
src/cox.jl	99.23% <100.00%> (+8.82%)	:arrow_up:
src/estimator.jl	100.00% <0.00%> (ø)
src/eventtimes.jl	100.00% <0.00%> (+7.69%)	:arrow_up:
src/kaplanmeier.jl	100.00% <0.00%> (+40.00%)	:arrow_up:
src/nelsonaalen.jl	100.00% <0.00%> (+50.00%)	:arrow_up:

---

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 9135253...0fc6e0a. Read the comment docs.

[andreasnoack]

I've addressed the comments but also bumped the minimum Julia version to 1.2 to avoid very old versions of Optim which made the results slightly off.