firebase-ios-sdk

Prepare for installed sub-projects (#1459)

* Ignore vim backup files

* Prepare for installed subprojects

This adds FIREBASE_BINARY_DIR to point to the out-of-source build
directory (which previously was passed around as FIREBASE_INSTALL_DIR).

This repurposes FIREBASE_INSTALL_DIR to point to
${FIREBASE_BINARY_DIR}/opt, and installation root for subprojects that
have an install step.

* Allow download directory to be specified