Allow to pass a chain cert while exporting to pkcs12 format
Corrected export to PKCS12 format, fix #33

Beware: this modification breaks earlier calls in order to ensure you
won't get some weird behavior with a broken PKCS12.

It adds the option to pass the PEM private key password, which wasn't
possible before.

Also, the name "pkey_pass" was misleading, as "pkey" usually stands for
"private key".

Replacing the "pkey_pass" par "in_pass" makes it easier to understand,
and "out_pass" does the trick for the PKCS12 password.
updated readme for pkcs12 export
Fix unit tests
Merge pull request #34 from EthACKdotOrg/pkcs12-modifications

Pkcs12 modifications