key loader and serialisation updates.

Priv model now supports PEM loading and serialisation:

from_pem(), as_struct(), as_asn1() and as_pem() methods added to Priv model.

Priv.from_asn1() behaviour changed. Now it returns Priv instance by
default. To obtain keystore, pass second argument to from_asn1()
function.

Field object now supports serialisation to LE.

Curve object now has method as_struct().
Box loading

Box.load() method added. It is now possible to add data into
running box instance.

Box constructor as well as load() method now support PEM keys and
certs and Priv and Certificate objects, not only fs paths.