Use libb64 instead of OpenSSL to encode/decode BASE64

Use libb64 instead of OpenSSL to encode/decode BASE64

Some crypto libraries (like CyaSSL, MatrixSSL and CommonCrypto) either
don't have or expose this functionality at all, expose only part of it,
or (like OpenSSL) have heavyweight API for it. Also, for the task as
easy as BASE64 encoding and decoding it's much better to use small and
simple specialized library.