Adds ES256, ES384, ES512 signer support

Support for ES256, ES384, ES512 signer.
On of the following PHP version required:
php-5.4.26
php-5.5.10
php-5.6.0

The ASN.1 parser from phpseclib is needed for the detection of the curve. In future versions of PHP, this informations is provided by [openssl_pkey_get_details](https://github.com/php/php-src/pull/916).
It might be better to add phpseclib to the [suggest section](https://getcomposer.org/doc/04-schema.md#suggest) of the composer file and adding a check if the ASN.1 parser is present.

Solves #15 
I think we'll also drop php 5.4 at some point :)