Fixes several issues#44

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westende wants to merge 5 commits intoacarbone:masterfrom

TotalDesign:master

Commits on Dec 17, 2014

Patched plugin in order to properly link to relatively linked resources in CSS.
Robbert van 't Westende
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Commits on Nov 4, 2015

Merge upstream.
Robbert van 't Westende
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Fix for multisite.
Robbert van 't Westende
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Fix double rewriting.
Robbert van 't Westende
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Fix garbage collection. closes acarbone/AssetsMinify#43
Robbert van 't Westende
committed