Add placeholder main class
Upgrade to bitcoinj 0.12-SNAPSHOT
Ignore gradle build directory
Add dependency on slf4j
Add custom repository for bitcoinj 0.12 snapshot

Because bitcoinj snapshots are not regularly published anywhere, I've
built bitcoinj 0.12-SNAPSHOT from bitcoinj/master at revision a0d722 and
published it to an artifactory repository for convenience purposes, i.e.
so that conference attendees can build and run this demo easily, without
having to build bitcoinj from source themselves.
Configure Gradle application plugin

This allows users to build and run the demo application by typing

    ./gradlew run
Fix typo in .gitignore
Add readme