Allow overriding the Logger used by Proxy

[hmhealey]

Related to #162, I'd like a bit more control over the image proxy's log output so that we can route it through the same logging interface that's used by the rest of our server. That's not currently possible because the Proxy uses the global log.Logger instance, but it's easy enough to get this functionality by giving the Proxy it's own log.Logger instance. By default, it creates its own instance that matches the one in the log package. I'm hoping that this could be useful to others.

There's a few places where the global logger is still used, but that's inside functions that aren't part of the Proxy and in the httpservice.Cache implementations, so changing those would be more intrusive.

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[codecov-io]

Codecov Report

Merging #174 into master will increase coverage by 0.31%.
The diff coverage is 87.5%.

@@            Coverage Diff            @@
##           master    #174      +/-   ##
=========================================
+ Coverage   84.48%   84.8%   +0.31%     
=========================================
  Files           5       5              
  Lines         490     500      +10     
=========================================
+ Hits          414     424      +10     
  Misses         56      56              
  Partials       20      20

Impacted Files	Coverage Δ
imageproxy.go	88.42% <87.5%> (+0.64%)	⬆️

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[willnorris]

Looks great! Thanks @hmhealey