Fix API : functions to access fields properties

Add the ability to know :
- the signedness of an int
- the base of an int
- the encoding of an int
- the len of an array
- the encoding of a string

refs #139

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>