Variety of improvements

[brson]

Highlights:

• Installation components are each stored in their own directory, which simplifies tools that want to pick the installer apart again, and eventually will allow components to include the same file.
• Installs an uninstall.sh script.
• Adds a --components flag for selecting which components to install/uninstall, as well as a --list-components flag.
• Reduces output spew to a few lines unless --verbose is provided.
• Don't pass -f where it isn't needed.
• Consistently overwrite existing files while creating backups.
• Reorders some operations to improve failure recovery.
• Adds a log of actions taken during installation for troubleshooting.

Output of installing rust currently looks like

install: creating uninstall script at /home/brian/.multirust/toolchains/rust-packaging/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'cargo'
install: installing component 'rust-docs'

    Rust is ready to roll

Closes #15, #13, #12, #9, #8, #6, #5

[rust-highfive]

@brson: no appropriate reviewer found, use r? to override

[brson]

The one thing I wanted but decided not to add because of complexity is 'negative component deps', which would allow multirust to uninstall existing rust's without causing havok.

[alexcrichton]

👾

[brson]

@alexcrichton For the record, no. The installer normally uninstalls everything it's previously installed before upgrading. The backups only happen in exceptional circumstances when some other agent has put files in our way.