formula_installer: add fetch_dependency method.
formula_installer: add fetch_dependency method.

This uses `FormulaInstaller#fetch` to recursively fetch dependencies
through the dependency tree and allows `install_dependency` to not
do any fetching.

vitorgalvao/brew,Homebrew/brew,Homebrew/brew,vitorgalvao/brew,sjackman/homebrew,Homebrew/brew,JCount/brew,EricFromCanada/brew,konqui/brew,MikeMcQuaid/brew,sjackman/homebrew,reitermarkus/brew,JCount/brew,mahori/brew,konqui/brew,sjackman/homebrew,MikeMcQuaid/brew,JCount/brew,claui/brew,mahori/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,claui/brew,nandub/brew,claui/brew,reitermarkus/brew,sjackman/homebrew,vitorgalvao/brew,EricFromCanada/brew,nandub/brew,EricFromCanada/brew,mahori/brew,mahori/brew,JCount/brew,MikeMcQuaid/brew,reitermarkus/brew,vitorgalvao/brew,konqui/brew,nandub/brew,reitermarkus/brew,claui/brew,konqui/brew,EricFromCanada/brew