Darwin support

[turbolent]

After running into issues like #691, I finally got Tigerbrew running on Darwin 8.01 (which corresponds to Mac OS X 10.4).

I'm not sure if/how it is possible to add support in a non-breaking way, but here are the changes that worked for me:

• Remove the /System/Library/Frameworks/Ruby.framework/Versions/Current prefix in the top two lines of https://github.com/mistydemeo/tigerbrew/blob/master/Library/ENV/4.3/cc#L1-L3.

  Darwin does not provide /System/Library/Frameworks/Ruby.framework, it is just installed as /usr/bin/ruby

• Return true from OS::CLT::installed? in https://github.com/mistydemeo/tigerbrew/blob/master/Library/Homebrew/os/mac/xcode.rb#L171.

  Darwin comes with developer tools like gcc, so Xcode or CLT are not necessary.