#25 Implement SXT key container

[Alexander Vdolainen]

Design and implement container to store a sxt keys.
implement import/export functions.

Depends on [#37]

[Alexander Vdolainen]

for file decide to use the form (currently ed25519 supported only)
(ppkp-ed25519 "<base64>")
no, will not use sexpr for parsing (but there are still possible to do it from other application, that's why s expression used).
within a base64 will store some random numbers, encrypted key (if applicable), and crypt info (if applicable).
don't commit anything yet, because of testing process and some implementation issues.</base64>

stay tuned :)