Export public key for external usage: ASN1 (Java supported key)

[hatemalimam]

As discussed here I'm opening this PR.

Please review the code, at my backend part it's working like a charm.

Thanks.

[amitaymolko]

Thanks for taking the time to contribute!

Looks good!
But, you forgot to add Android support for that function.
We can't have this returning an error on Android.

Thanks!

[hatemalimam]

Firstly I thought the android part is generating a sane key,
so I thought to call getPublicKey in the newly introduced method,
but strangely testing the android key, it's missed up as the iOS one !
I didn't expect that, I'm inspecting the use of spongycastle...