Snowplow Java Tracker Core v0.1.4

Various important bug fixes; also note that this version of core is built against Java 1.6.

• Fixed incorrectly casts timestamp from long to double (#84)
• Moved 'eid' from Subject to Tracker class to prevent duplicated event IDs (#95)
• Now setting source/targetCompatibility to 1.6, thanks @dstendardi! (#94)
• Fixed typos in Tracker preconditions, thanks @czchen! (#99)