Implement serde traits for Signature

Update serde to 1.0

add compact signature encoding

Use safe copy_from_slice() rather than unsafe copy_nonoverlapping()

Implement From<[u8;32]> for Message

Improve Error::InvalidMessage comment

Use DefaultHasher in place of the deprecated SipHasher

Annoyingly, this is a major version bump since we expose libc types in the FFI module

Turns out you cannot initialize constant SecretKeys in any way; these
two constants should cover most sane use cases (other good choices
are the SECG generator and the Alpha CT generator, but these will
wait for a major CT-supporting upgrade, unless demand for them appears.)

We had added a necessary include directory to build.rs but not updated the
library on crates.io, so dependencies of rust-secp were failing to build.
(However, on my local system I had secp installed in /usr/local/include,
so I did not notice the problem until Travis pointed it out on a different
project!)

Pieter moved some stuff I need into the contrib/ directory which does
not expose anything through the shared lib, so I need to statically
link.

I might also use this to do evil things to expose the SHA256 code
in libsecp, but not for now ;).