This is the first cut of the Nettle integration for your review.

Notes:

The tests are inlined for convenience but will be in a separate file
later.

This patch implements, with tests:

- meta info about Nettle's exported hashes and ciphers

- all the hashing functions exported by Nettle

- HMAC keyed hashes with all the hashing functions above

- encrypt/decrypt with all the ciphers exported by Nettle in ECB, CBC, and CTR modes

- PBKDF2 with md5, sha1, and sha256 hashing (the generic version is not
  ready, and only sha1 is explicitly tested; the commented-out code will
  be rewritten soon)

Not done yet:

- RSA verify data signature: code looks OK; test cases NOT READY

- RSA verify digest signature; RSA sign data and sign digest

- DSA

- ECC

- GCM cipher mode

- Yarrow randomness

- PBKDF2 testing of at least md5 and sha256 hashing

I would appreciate any comments at this early stage.

Ted