Added optional user_agreement key. Displays text that end users can agree or not before continuing with install
added more options for changing text, title and button text.

work in progress
Better support for alert with dict for specifying alert values.
changed pkginfo item from 'alert' to 'preinstall_alert'
moved preinstall alert code to it's own function (takes preinstall_alert dict as argument)
compliant use of NSLocalizedString so genstrings can do its thing
changes to allow MSC compile in xcode 6
Revert "changes to allow MSC compile in xcode 6"

This reverts commit 13cd0d2caf5dc44054235aaca78f03a2b6e66ca9.
added pre uninstall capability as well
Reverted to preinstall_alert and preuninstall_alert to keep in the scheme of other pkginfo keys
Merge pull request #395 from bartreardon/user_agreement

Add optional preinstall_alert key to pkginfo plist.