Initial implementation of push_back() and push_back_empty().
Add javadoc comments for push_back() and push_back_empty().
Add variants of release() that take start and end positions.
Updated NEWS.
More useful exception message.
Rename range-based release() to release_range().

Mostly to avoid naming clashes and associated surprises with the variant
of release() that also takes 2 arguments (position and value reference).
Set the version to 0.10.3.