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.
I'm all for modularizing where it makes sense (eject the wallet today!). But the consensus logic by nature is an ugly atomic blob. There's nothing to be gained by modularizing it.