Added the add2virtualenv command
Renamed add2virtualenv to addpath; added to README
Added fish 2.x requirement to docs. Closes #14.
Moved auto activation into its own plugin. Closes #15.
Add auto activation plugin to installation instructions
Update description, add contributors
Use wheel for global requirements if it's enabled.
Refactor auto-activation
Replicate the original workon behaviour

executing workon without parameters returns a list of virtual
environments
Merge pull request #18 from alazaro/workon-list-envs

List virtualenvs when workon compat alias called without an argument
Add cdvirtualenv compat alias and cdpackages command.

Closes #19.
Get rid of "test: Missing argument" message when auto-activation enabled
Merge branch 'master' of https://github.com/adambrenecki/virtualfish
Document new commands from 4cf069
Workaround fish-shell/fish-shell#597. Closes #20.