trackScreenView schema incorrect

trackScreenView schema should put "/screen_view/" in stead of "/context/"
Remove guava as a dependency.

The only thing used from guava was Preconditions.java. With this, that file is copy/pasted and Android consumers are happy to be under the dex limit again.

Fixes: https://github.com/snowplow/snowplow-java-tracker/issues/86
Replace Map in trackUnstructed to SchemaPayload
Added deprecated on SchemaPayload 'add' methods
Prepped for Core 0.2.0 release
Merge branch 'lixiaoyi-master' into release/core-0.2.0
Merge branch 'hamidp-hamidp/remove-guava' into release/core-0.2.0
Updated CHANGELOG to include Vagrant work
Added dedicated Vagrant setup (closes #106)
Updated guidance message
Moved platform setting out of Subject into Tracker (#103)
Made setSubject method on Tracker public (closes #109)
Tidied up approach to schema constants in Tracker (closes #110)
Added Quickstart section to README (closes #111)
Added warning that Version.java is auto-generated (closes #112)
Added warning that Version.java is auto-generated (closes #113)
Incremented versions
Finalized release dates
Merge branch 'release/core-0.2.0'