Remove the support for doing anything other than copying
Handle clearing of an existing virtual environment ourselves
Merge pull request #1 from ionelmc/rewrite-windows-fixes

Windows fixes and whatnot
Move platform specifics into a "system" package. The builder now takes the system implementation as an argument.
Rename to flavours.
Use objects so we can have properties or other nice stuff.
Extend flavours and use them in the builders.