Adds scripts/install_arc.sh, which installs or updates arc and our project-specific dependencies.
Adds scripts/install_arc.sh, which installs or updates arc and our project-specific dependencies.

Summary:
The script can be run to do any of the following:

  # Install arc and dependencies and add arc's path to a user's shell setup script.
  # Just install arc and dependencies, letting the user configure their own paths.
  # Update an existing installation from a previous run of the script.

Reviewers: featherless, #mdc_ios_owners

Reviewed By: featherless, #mdc_ios_owners

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D289

chriscox/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,samnm/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios