A bunch of changes (v2)

This request contains the same changes as #43 but it has been updated to apply cleanly to the latest head.

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I decided to make `rpi-update` behave more like how I wanted, so I thought I'd share.  This pull request includes the following changes:

* Readme updates and restructuring, based on Bengt's #20
* Merged adammw's #14 to fix a possible `BOOT_PATH` and `ROOT_PATH` issue **(this is already merged into HEAD)**
* Show a bunch more progress messages from different steps, including the ones from lacop's #17 and more
* Show output from `git`, `wget`, `cp` - this helps to diagnose whether the update process is hung up or if it is still continuing
* Prefix script messages with `***` and errors with `!!!` to make it clear which output comes from the script and which comes from external commands
* Add and document option to disable self-updating (`UPDATE_SELF=0 rpi-update`)

The commits aren't as cleanly separated as I'd like.  If you don't want all of this stuff, let me know, and I'll try to clean up what you do want.