Conda

* revert 2217 closing request sessions, 2233

* adds a `conda clean --all` feature, 2211
* solver performance improvements, 2209
* fixes conda list for pip packages on windows, 2216
* quiets some logging for package downloads under python 3, 2217
* more urls for `conda list --explicit`, 1855
* prefer more "latest builds" for more packages, 2227
* fixes a bug with dependency resolution and features, 2226

* fixes track_features in ~/.condarc being a list, see also 2203
* fixes incorrect path in lock file error 2195
* fixes issues with cloning environments, 2193, 2194
* fixes a strange interaction between features and versions, 2206
* fixes a bug in low-level SAT clause generation creating a
preference for older versions, 2199

* fixes an install issue caused by md5 checksum mismatches, 2183
* remove auxlib build dependency, 2188

* The solver has been retooled significantly. Performance
should be improved in most circumstances, and a number of issues
involving feature conflicts should be resolved.
* `conda update <package>` now handles depedencies properly
according to the setting of the "update_deps" configuration:
--update-deps: conda will also update any dependencies as needed
to install the latest verison of the requrested
packages. The minimal set of changes required to
achieve this is sought.
--no-update-deps: conda will update the packages *only* to the
extent that no updates to the dependencies are
required
The previous behavior, which would update the packages without regard to
their dependencies, could result in a broken configuration, and has been
removed.
* Conda finally has an official logo.
* Fix `conda clean --packages` on Windows, 1944
* Conda sub-commands now support dashes in names, 1840

--------------------
* improve handling of local dependency information, 2107
* use install.rm_rf for TemporaryDirectory cleanup, 3425
* fix the api->conda substitution, 3456
* error and exit for install of packages that require conda minimum version 4.3, 3726
* show warning message for pre-link scripts, 3727
* fix silent directory removal, 3730
* fix conda/install.py single-file behavior, 3854