More features and associated cleanup
+ Return more descriptive exceptions for standard failure states
+ Determine if module is being run inside a virtualenv and attempt to use the virtualenv's version of virtualenv
+ Redirect STDERR to build.err
+ Simplify logging (still work to do, _create and _execute are very similar)
+ Homogenize error handling between install and uninstall (more refactoring possible, install and uninstall are very similar)