Add RSA-SHA1 signatures

RSA keys should be given in a PEM format. Keys are read using
BouncyCastle's PEMReader [1].

Tests use a 512b key generated with OpenSSL.

[1]: http://www.bouncycastle.org/docs/pkixdocs1.5on/org/bouncycastle/openssl/PEMReader.html
Merge pull request #27 from jstepien/rsa-sha1

Add RSA-SHA1 signatures