When creating an environment write activation scripts into $PREFIX/bin.
An activation script, when sourced, adds $PREFIX/bin to $PATH and adds
'deactivate-lua' function that removes it. Activating an environment
automatically deactivates previous one.
Supported shells:
* bash, zsh, dash: $PREFIX/bin/activate
* fish: $PREFIX/bin/activate.fish
* csh, tcsh: $PREFIX/bin/activate.csh
Also fixed compilation error for location paths containing double quotes.
TODO: add activation scripts for Windoss (powershell and batch).
Ref #23.
