Enable windows builds on travis

bump version to 0.12

remove `ops::Index` impls for `Signature`

Various slice-handling ergonomic improvements

add `ThirtyTwoByteHash` hash trait which can be implemented for easier conversion of things to `Message`s

This makes more conceptual sense and does not add any new error paths, since even
valid `SecretKey`s were able to be invalid tweaks.

Update upstream libsecp; remove `without_caps` and all use of dummy contexts

export `serde`, `rand` and `libc`; update version to 0.11.5

Increase version to 0.11.4

add `FromStr` implementation for key types

fix build.rs on Windows

bump version to 0.11.3

attempt to fix docs.rs docs

bump version to 0.11.2

Use correct flag on big endian machines.

This will fix `tests::sign` test case on mips/mips64. Verified with
`cross`[1] tool.

    cross test --target mips-unknown-linux-gnu
    cross test --target mips64el-unknown-linux-gnuabi64

Unfortunately this fix doesn't make `tests::test_debug_output` test case
pass, but this is about debug output so its not as critical as this
patch. See #63 for a discussion.

[1]: https://github.com/japaric/rust-cross

Revert "remove PublicKey::combine"

This reverts commit 17450e91.