Snowplow Java Tracker v0.2.0

Various improvements to the underlying codebase:

• Blog post

API changes

• Removed Tracker base constructor (#23)
• Changed transaction_items to use a Class, not a Map (#31)
• Removed context_vendor from Tracker constructor (#36)
• Camelcased all functions (#20)

Codebase updates

• Renamed PlowContractor (#19)
• Replaced org.JSON code with Jackson 1.9.13 (#14)
• Made sure null fields are left off from querystring (#21)
• Made track_ecommerce_transaction_item private (#29)
• Removed event_name and event_vendor from trackUnstructEvent (#32)
• Created Constant class or equivalent (#33)

Testing

• Moved all 3 main()s into unit tests (#5)
• Added Travis support to README (#9)
• Added tests for track_ecommerce_transaction and _transaction_item (#25)