crypto/algorithms/base.js: Convert to es6

Convert base to an es6 module with es6 classes, for clarity and to help with
jsdoccing.

Complications are:

* jsdoc gets confused by `export class`, so the exports are separated.

* turns out that extending Error is a bit difficult, so instanceof doesn't work
  on derived Error classes. This only really affects us in one place (app-side
  code shouldn't be doing instanceofs anyway), so just use `name` instead.

src/crypto/index.js

@@ -1121,11 +1121,6 @@ Crypto.prototype._signObject = function(obj) {
     obj.signatures = sigs;
 };
 
-/**
- * @see module:crypto/algorithms/base.DecryptionError
- */
-Crypto.DecryptionError = algorithms.DecryptionError;
-
 
 /** */
 module.exports = Crypto;