Merge tag '0.17' into develop

Lower threshold for experiments by removing generation of Sample.h from CMakeLists
Added link to wiki
Added link to container backend
Adjusted type_traits to compile with clang-3.1, too

The new version is cleaner anyway, since it can be used with any type
now.
Fixed type trait code
Fixed tvin-handling in assignment
Unified tvin/trivial_value_is_null usage for =,==,!= and !
Make sure that tvin is used in the allowed places only
Use _traits in column specs
Made can_be_null a recursive trait
Transport can_be_null into select pseudo table
Follow outer tables through the statement.

This is important to determine which result column of a select can be
NULL.
Use potential outer table dependencies in pseudo tables

The dependency indicates whether a column of a pseudo table can be null.
Added a few reminders which expressions can be null as well
Transport can_be_null into subselect used as a result column
Transport null_is_trivial_value and can_be_null to result fields
Using column's can_be_null and connector's enforce_validity
Renamed field to field_spec
Renamed _result_entry_t to _result_field_t
Fixed type error in join
Added testability and tests for can_be_null and null_is_trivial in result entries
Aligned structure of connector traits with other traits