GH-21 and GH-104 jdk8 support and move of the xdm IO Events Model here as a new module

Description

downgrading the java compiler's source and target to support jdk8 users
the XDM Adobe I/O Events Model has find here a new home and a new name
- relocating com.adobe.event:xdm-event-model as a new module of this repo
- renaming it to com.adobe.aio:aio-lib-java-events-xdm
other minor updates:
- aligning with the relocation of The artifact junit:junit-dep:jar:4.11 to junit:junit:jar:4.11
- polishing: adding a few README.md details
- fixing typo