Added ability to pass the gradient function to Adadelta

[alansaul]

Adding ability to pass along the gradient function to use for Adadelta, necessary for doing stochastic gradients with GPy models.

[codecov-io]

Codecov Report

Merging #24 into master will decrease coverage by 0.09%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master      #24     +/-   ##
=========================================
- Coverage   96.68%   96.59%   -0.1%     
=========================================
  Files          26       26             
  Lines        2084     2087      +3     
  Branches      332      333      +1     
=========================================
+ Hits         2015     2016      +1     
- Misses         55       56      +1     
- Partials       14       15      +1

Impacted Files	Coverage Δ
paramz/optimization/optimization.py	98.4% <50%> (-1.06%)	⬇️

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[mzwiessele]

Why is the coverage going down here? Do we have the ability to put a test in place, which tests the stochastic gradient function?

[mzwiessele]

We need to put in the test case here! Then we can merge :)