chore(example/invoice): async demo + multiple process instances

- add async demo
- start multiple process instances

related to #CAM-2800
fix(example/invoice): fix test case of invoice example

related to #CAM-2800
fix(qa): fix content-type comparison if q parameters is also returned

- happens on websphere and wink

related to #CAM-2748
chore(engine): use VariableMap in impl classes

related to #CAM-2979
chore(engine): use VariableMap in case instance builder
fix(qa): adjust webapps IT to modified invoice example

related to #CAM-2800
chore(engine-rest): remove unused interface
chore(pom): add spin/connect plugin and adjust dependecies

related to #CAM-3061