Updated plan to use an instruction mapping instead of an if statement

This is the next step in #997 

cc @asmeurer @ilanschnell 
```
Traceback (most recent call last):
  File "/Users/travis/anaconda/bin/conda-env", line 7, in <module>
    execfile(__file__)
  File "/Users/travis/Dropbox/work/github/conda/conda-env/bin/conda-env", line 5, in <module>
    sys.exit(main())
  File "/Users/travis/Dropbox/work/github/conda/conda-env/conda_env/cli/main.py", line 51, in main
    return args_func(args, parser)
  File "/Users/travis/Dropbox/work/github/conda/conda/conda/cli/main.py", line 207, in args_func
    args.func(args, p)
  File "/Users/travis/Dropbox/work/github/conda/conda-env/conda_env/cli/main_remove.py", line 73, in execute
    plan.PREFIX: prefix,
AttributeError: 'module' object has no attribute 'PREFIX'
```