libsecp256k1 integration

@sipa suggested I go ahead and PR this to get discussion going. It's his work, I've just integrated it into our build-system.
This was done in a few stages:
- Add libtool dependency. I've tried to update all relevant docs, but very possible I missed some. Note that we don't actually make real use of libtool yet, it's only used when linking the subproject.
- Pull in libsecp256k1 subtree
- Add configure option "--enable-libsecp256k1". This is disabled by default.
- Update the makefiles to link against the lib where necessary
- Finally, add in the actual implementation.
Should secp256k1_ecdsa_privkey_export really NEVER return NULL in ordinary circumstances here? If there's a normal condition it might, this shouldn't be an assert...