Move the license to a separate file and use an SPDX identifier in the
source file. This is more terse but with the same legal coverage.

Keep Clark's name as author but add "the rust-bitcoin developers" also
for attribution and accountability.

Add whitespace, and use second layer markdown header for subheading.

As we are doing throughout the rust-bitcoin ecosystem; bump the MSRV to
Rust 1.48.0

We do not have any unknown lints, I'm not even sure its possible to get
them now that we have the MSRV in the clippy config file.

Clippy emits:

 warning: trait objects without an explicit `dyn` are deprecated

We are currently configuring the linter to allow this but it is not
necessary.

Use `dyn` as suggested by the linter and remove
`allow(bare_trait_objects)`.

Currently we are setting a bunch of lint config options to their
default values, this is redundant.

Use the recently learned about `doc_auto_cfg` feature and enable it
using a custom `docsrs` compiler conditional configuration option.

Fix and improve the rustdocs while we are at it.

Add docs build to CI script but do not enable it in the github actions
yet.

Fix: rust-bitcoin#75

Improve the CI pipeline by doing:

- Run docs build with stable/nightly as required
- Run 32 bit tests (default features with a 32 bit target)
- Run cross tests (default features with a s390x target)

Add a `githooks` directory and a `pre-commit` hook. Add a section to the
readme instructing devs how to configure git to take advantage of the
githooks.