Handle invalid path : missing member name after .

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>
path: compile and query path '$' to retrieve root node

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>
Handle invalid path: invalid first character

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>

Conflicts:
	json-glib/json-path.c
Add tests for json paths (2 invalids, 1 valid)

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>

Conflicts:
	json-glib/tests/path.c