rest: renamed DefaultDataSourceApi to DataApi
data: added JsonPointerSerializer

Using the jackson ToStringSerializer does not work for jersey unfortunately
...
rest: added DataSourceApi for viewing configured sources
rest: added AbstractApi for handling base URL
rest: added FilterChainApi
data: type of url in DataSource is now java.net.URL instead of String
data: DataSource has json ObjectNode property which contains schema

Schema here means the data schema that fits that which can be downloaded
by ODS clients.
rest: added addition and deletion of sources