JsonTree: add documentation, rename methods

Rename .pushd() to .append(), .popd() to .pop(). Using .pushd() was confusing
for people used to what pushd does in a shell.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
JsonSchemaTree: add Javadoc

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>