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.
Including it as subtree is better IMO. It leaves us control over the code (which is one of the reasons for moving away from OpenSSL in the first place!), and avoids that the person building it has to install a dependency that's pretty much unknown right now.