Conda

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* fix 'conda update' taking build number into account

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* allow packages from create_default_packages to be overridden from the
command line
* fixed typo install.py, issue 566
* try to prevent accidentally installing into a non-root conda environment

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* conda update: don't try to update packages that are already up-to-date

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* improve the speed of clean --lock
* some fixes to conda config
* more tests added
* choose the first solution rather than the last when there are more than
one, since this is more likely to be the one you want.

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* fix detection of prefix being writable

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* bug: not having track_features in condarc now uses default again
* improved test suite
* remove numpy version being treated special in plan module
* if the post-link.(bat|sh) fails, don't treat it as though it installed,
i.e. it is not added to conda-meta
* fix activate if CONDA_DEFAULT_ENV is invalid
* fix conda config --get to work with list keys again
* print the total download size
* fix a bug that was preventing conda from working in Python 3
* add ability to run pre-link script, issue 548