Two new commands: list and uninstall

[guest]

Suggested by Claus Reinke:

    .. at the very least I would hope to see the following two
    additional commands:

    list            List all installed packages
    uninstall    Provide uninstall information

    the former ought to list known Haskell implementations and their
    package databases, the latter ought to output status information
    about the (current) package (has it been registered anywhere? if
    yes, point to the unregister command first. have files been copied
    anywhere? if yes, explain which directories can simply and safely
    be removed).

[duncan]

Note that we can only do this for Haskell implementations that have any notion of installed / registered packages.

[duncan]

We should implement this in cabal-install. There is already a list command for cabal-install but it lists uninstalled packages. Probably it should list all packages, installed and otherwise with an indication of whether they are installed or not. There should perhaps also be a flag to ask to list just installed.

[duncan]

Uninstall is tricky because currently we do not keep any record of what files we install or where we install them.

[duncan]

Split into two tickets of rather different difficulty #235 for the list feature and #234 for tracking installed files to allow for later uninstall.