Add pretty package output for 'conda info package'

This replaces the mostly useless output that existed before. I also removed
the conda info path functionality since it duplicates conda package -w.
Add back json api to conda info package
Don't allow to remove the current environment

Fixes #639.
Improve error message for forgetting to use source with activate and deactivate

Fixes #601.
add new section in changelog
Make find_executable find the actual executable, not conda-executable

This is the same find_executable() from conda-build except with an option to
not include the root prefix.
Only show the conda install binstar hint if binstar is not installed
Merge branch 'master' of github.com:conda/conda
Fix undefined variable