Inconsistent license infomation for asn1 and x509

The files in the `asn1` and `x509` directory are taken from the Go standard library, which is licensed under a BSD-style license. They have license headers that say:
```
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
```

But the [license file in this repo](https://github.com/google/certificate-transparency-go/blob/master/LICENSE) has the Apache License, Version 2.0, which is not the BSD-style license referred to.

Either the correct license file should be added to those directories, or these file should be relicensed to Apache 2.0 by Google.

Compare the license headers in the asn1 and x509 directories with the repository LICENSE file. Before changing either, seek a project-owner decision on whether to add the referenced BSD-style license file or relicense the copied files; done means the headers and applicable license documentation are consistent.

Written by the indexing model from the issue text.