Fix symmetry testing

[rcannood]

This is a fix for #3. The first commit (db9f60f) add tests to verify whether proxyC produces the correct results, even when x and y have the same dimensionality. The second commit (7df91ee) fixes the problem by adding a boolean argument that allows to specify whether the output matrix will be symmetric.

[rcannood]

Thanks for merging #2! I merged koheiw/proxyC@master into my branch, in order to enable travis for this PR.

[codecov-io]

Codecov Report

Merging #4 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master       #4     +/-   ##
=========================================
+ Coverage   92.85%   93.16%   +0.3%     
=========================================
  Files           4        4             
  Lines         266      278     +12     
=========================================
+ Hits          247      259     +12     
  Misses         19       19

Impacted Files	Coverage Δ
src/pair.cpp	83.65% <100%> (ø)	⬆️
R/proxy.R	98.68% <100%> (+0.24%)	⬆️
src/linear.cpp	98.41% <100%> (ø)	⬆️

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