Conda

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* simplify call to menuinst.install()
* add menuinst as dependency on Windows
* add ROOT_PREFIX to post-link (and pre_unlink) environment

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* improve conda clean when user lacks permissions, 1807
* make show_channel_urls default to True, 1771
* cleaner write tests, 1735
* fix documentation, 1709
* improve conda clean when directories don't exist, 1808

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* fix bad menuinst exception handling, 1798
* add workaround for unresolved dependencies on Windows

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* allow explicit file to contain MD5 hashsums
* add --md5 option to "conda list --explicit"
* stop infinite recursion during certain resolve operations, 1749
* add dependencies even if strictness == 3, 1766

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* added a pruning step for more efficient solves, 1702
* disallow conda-env to be installed into non-root environment
* improve error output for bad command input, 1706
* pass env name and setup cmd to menuinst, 1699

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* add "conda list --explicit" which contains the URLs of all conda packages
to be installed, and can used with the install/create --file option, 1688
* fix a potential issue in conda clean
* avoid issues with LookupErrors when updating Python in the root
environment on Windows
* don't fetch the index from the network with conda remove
* when installing conda packages directly, "conda install <pkg>.tar.bz2",
unlink any installed package with that name (not just the installed one)
* allow menu items to be installed in non-root env, 1692