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.
But something could be said that secp256k is in exactly the same spot as leveldb. It's critical to the consensus.
Sure, @sipa is upstream here which helps, *but* by including the code we avoid having to take version mismatches into account, for example. We test with one bitcoind-secp256k-leveldb combination and everyone will use that.