Added working version of exchange-sdk upgrade to release to maven-central.
Added /target to ignore.
Updated topmost gitignore. Removed gitignore from engine.
Merged changes from dev. Removed jar files before release.
Updated Maven repository from test to prod.
Incremented version for next iteration 0.11.1 -> 0.11.2-SNAPSHOT. Updated javadoc.
Set SDK dependency version to 0.11.2-SNAPSHOT. Updated exchange demo with image attaching functionality.
Updated exchange tests to use current dev SDK version 0.11.2-SNAPSHOT.
Merge branch 'dev' into maven_central
Merge pull request #41 from OfficeDev/maven_central

Merged latest changes as committed to Maven Central. Incremented Exchange SDK version for new iteration (0.11.2-SNAPSHOT). Updated Exchange samples and e2e tests accordingly.