Fix git download strategy

A formula can have just a @head url or the user can specify to install HEAD
with --head. We support subversion and git checkouts.

The version is set to HEAD for head builds.

Next step is making brew update handle these installs correctly.

Return nil instead

We'd gotten to the stage where Formula was so lean, it was pointless to
separate it.

Untested

Adds a new DownloadStrategy that checks files out from Subversion to the
cache, then exports over to the temp folder for the build.

To use checkout with the svn:// protocol or reimplement download_strategy and
return SubversionDownloadStrategy.

This patch adds a ArchiveDownloadStrategy that handles downloading
tarbarlls and decompressing them into the staging area ready for brewing.

Refactored safe_system and curl into utils.rb

Signed-off-by: Max Howell <max@methylblue.com>

Modifications to Adam's original patch:

I reverted objectification of checksum verification because I couldn't think
of any other download validation methods that might be useful to us in the
future, so allowing such flexibility had no advantages. If we ever need this
to be OO we can add it. But for now less complexity is preferable.

I removed the @svnurl class member. Instead download_strategy is autodetected
by examining the url. The user can override the download_strategy in case this
fails. Thus we already can easily add support for clones of git repositories.

Signed-off-by: Max Howell <max@methylblue.com>

Automagically determines prefix to install to for DIY installations.

README amended.

Because directories don't have extensions.

Included test

Signed-off-by: Max Howell <max@methylblue.com>

I agree with Adam, this makes sense.

The default is p0, but if you return a Hash instead of an array from patches you can specify patch level. See the comment documentation.

Closes Homebrew/homebrew#10

Closes Homebrew/homebrew#13

Strange that they did often enough for my tests to work.

It was raising an exception, which stopped prune in its tracks when
something was installed root, eg. Fuse.

Speeds up Python formula plenty in clean phase

libpng and that are in the X11 formula

eg. http://github.com/lloyd/yajl/tarball/1.0.5