Switch from wget to curl for downloads

May solve issues like https://github.com/Hexxeh/rpi-update/issues/65
Download firmware without using git

This removes the requirement for git. We never make use of partial git updates,
and it is quicker to download the tarball with curl than use git.

Also piping the curl output into tar is quicker than downloading then un-tarring.

Remove the verbose options from the copies (they don't fail, slow down the copies
and swamp any more useful output messages).

By default delete /root/.rpi_firmware (as it's easy to recreate). Use
SKIP_REPODELETE=1 if you want to keep it.

Add option SKIP_BACKUP=1 if you don't want a backup of old firmware and modules.
Update readme for latest version of rpi-update
cosmetic: Make use the REPO_API variable
Support install from local repo.

See: https://github.com/Hexxeh/rpi-update/issues/134
Small header tidyup
Merge pull request #139 from lurch/patch-1

Small header tidyup
Update README.md

Upon installation of the tool, the command executed writes rpi-update to /usr/bin detailing a 301 error, indicating the source of the update has changed to https://raw.githubusercontent.com/Hexxeh/rpi-update/master/rpi-update. This address change has been reflected in the proposed file change.
Merge pull request #144 from kcl-lescalan/patch-1

Update README.md
Update url to githubusercontent
Merge branch 'master' of github.com:Hexxeh/rpi-update

Conflicts:
	README.md
	rpi-update