Update dependencies and shade cassandra-all usage.

Update dependencies and shade cassandra-all usage. Astyanax is deprecated but still used by a lot of projects and Astyanax's very old dependencies have been making it difficult to migrate projects from Astyanax to the DataStax driver. The migration can be especially difficult for projects that need both Astyanax and DataStax drivers to exist for some time (e.g. an app that wants to migrate its own Astyanax dependency to DataStax driver but still has other transitive dependencies to Astyanax).

• Shaded cassandra-all to avoid dependency incompatibilities with newer versions of cassandra-unit/CassandraDaemon/EmbeddedCassandra for unit/integration testing. Relocates astyanax-cassandra's "org.apache.cassandra" dependencies to "com.netflix.astyanax.org.apache.cassandra."
• Updated gradle wrapper to 4.1
• Updated all explicit dependencies to newer compatible versions.
• Tested with Guava versions 19-25.
• Eliminated/refactored some dependency declarations to reflect true dependencies and minimize transitive dependencies forced on Astyanax users.
• Moved dependencies not actually used by src/main to "testCompile" to keep them out of user's transitive dependencies.
• Added missing license headers.
• Updated readme to more explicitly suggest Astyanax users migrate to DataStax driver.
• Note that with shading cassandra-all, currently IntelliJ will incorrectly render "com.netflix.astyanax.org.apache.cassandra." as unfound. This is a known IntelliJ bug. Astyanax still builds perfectly fine via Gradle. See README.markdown for more info.