Extension dependency management

A number of extensions now depend on other extensions. To name a few, CiviVolunteer depends on org.civicrm.angularprofiles, nz.co.fuzion.relatedpermissions depends on nz.co.fuzion.entitysetting, and com.aghstrategies.statelegemail depends on some extension whose name I can't think of right now.

The desire is to support some way of expressing these dependencies so that they are either automatically downloaded or the user is prompted to download them before enabling the dependent extension.