[FLINK-1282] [docs] Update layout of docs

This closes #235.
[FLINK-1286] [APIs] [runtime] Fix serialization in CollectionInputFormat and generate meaningful error messages
[FLINK-1278] [runtime] (part 1) Remove special code paths for the Record data type in the input readers and the source task.
[Scala API] Case Class serializer can work with classes that cannot be instantiated empty
[FLINK-1282] [docs] Added {{ site.baseurl }} to css and image paths
[FLINK-1282] [docs] Make Flink logo link back to frontpage

This closes #237.
[FLINK-1234] Activate hadoop2 profile by default

This closes #232
deprecated current aggregation code
Test cases and API for new aggregations
ungrouped aggregations
grouped aggregations
missing license headers
moved AggregationUdf into correct package
name the aggregation operator in the DAG
additional tests for aggregation
CountAggregationFunction does not need an input type
documentation updates
fixed test case
fixed CI build
renamed old aggregation convenience functions