Add basic pinning functionality

If a file conda-meta/pinned is in an environment the lines of that file will
be added to the specs with every install. If the package is explicitly
included in the command line, it overrides the pin.

This does not yet affect conda remove.
Don't automatically override pinned packages

You can override all pinning with the --no-pin flag.
Merge branch 'master' into pin

Conflicts:
	conda/cli/install.py
	conda/plan.py
Check for pinning in conda remove
Merge pull request #640 from conda/pin

Add basic pinning functionality