SslServer: Add support for Elliptic Curve keys

SslServer: Add support for Elliptic Curve keys

If the key won't load as an RSA key, attempt to load it again as an
EC key.  DSA support was not added because DSA is obsolete and no-
one should be using it.

Note that this only works with Qt5.5 and up as EC support was added
in that version (https://github.com/qt/qtbase/commit/962ea569). An
if macro has been used to allow for continued compilation under Qt4
and Qt5<5.5.

Closes GH-344.

(cherry picked from commit d31101ed316b6449de0d8dad7a1e1e8d097807a5)