Drop commons-codec dependency

[emlun]

See #21. commons-codec is currently only needed because JRE 7 doesn't have java.util.Base64; this dependency can be dropped after JRE 7 reaches end of life in July 2019.

[emlun]

Seems like JRE 7 actually passed end of free public updates as early as 2015, so I guess we can actually do this whenever.

[emlun]

I know, I've implemented Base64 before, but I see no reason not to use java.util.Base64 when the JRE so graciously provides it.

[emlun]

I understand your concern, but we have decided to not support Java versions that have been abandoned upstream. You are of course welcome to fork the repository and support outdated Java yourself, if you like.