Fixes to work on Android

First commit is addressing https://github.com/jscep/jscep/issues/19
Second one is related to the problem that (at least on my Android devices) the _[sig]withRSA_ is not listed using
> private boolean algorithmExists(final String serviceType, final String algorithm)

when _[sig]WithRSAEncryption_ is (however you can still use _[sig]withRSA_ in _JcaContentSignerBuilder_). These algorithms have different OIDs, but BouncyCastle threats them the same:
http://www.bouncycastle.org/viewcvs/viewcvs.cgi/java/crypto/src/org/bouncycastle/ocsp/OCSPUtil.java?view=markup
http://crypto.stackexchange.com/questions/12973/what-is-the-difference-between-sha256withrsa-and-sha256withrsaencryption-digest