Changes in this version include:
* Aliases are now defined by relative symlinks rather than in the
  formulae themselves. Many commands are faster now that they don't
  have to read every formula to find aliases.

* "url" now supports the same features as "head", including ":using"
  and version specifiers for VCS systems.

* Files and methods marked for deprecation in 0.7 have been removed.

* The Formula DSL now supports "skip_clean :all" and "keg_only 'reason'"

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Specifically, it can accept all the VCS tag specs that head could,
making it more useful for defining stable versions that come from
VCS instead of tarballs.

A new "SoftwareSpecification" class was added to implement this.

This new class holds a "spec" for downloading a software package.
It combines the url (or head url) with the "specs" [1] that head has
been able to take.

This allows both the stable (url) and unstable (head) specification
for a software package to co-exist without stomping on each others
"specs".

[1] "specs" contain instructions on which branch/tag/revision/etc. to use
    from the source repository URL.