Zerotk.virtualenv-api

* 21: Converted `README.md` to an rST document to remove dependency on
`setuptools-markdown` (sjkingo)

* Added `upgrade_all()` function to upgrade all packages in an environment (sjkingo)
* Added `readonly` argument to constructing environment that can be used to prevent
operations that could potentially modify the environment (sjkingo)
* Added support for passing ~ to construct environment (e.g. `~user/venv`) (sjkingo)

* Version bump for broken PyPi release (sjkingo)
(no new changes to code)

* Improved search function that will return more accurate results. This
includes a breaking change where the package list returned by `env.search()`
is now a dictionary. (sjkingo)
* Prevent pip from checking for new version of itself and polluting the output
of some commands (sjkingo)

* Support for creating a wheel of packages (rmb938)

* Support changing the interpreter path (jlafon)
* Improve Unicode support in pip search that broke tests (jlafon)