moved `index` and its submodules into individual crates.

`encoding::index` module was semi-internal but still exposed;
this commit moves it to individual crates on which the main
`encoding` crate depends. they can be used without the main crate.

`encoding-index-*` crates are individually versioned according to
the Encoding Standard; any changes to the identifier would update
the version. for the monotonicity the crate version is based on
the date field, but it doesn't really affect the crate version.
bumped the version and added an `examples` directory.
updated .travis.yml to test the subcrates as well.
0.2.0: the removal of `index` module.

this does *not* mark the beginning of proper versioning (sorry!),
but it does signal the significant changes to the overall module
structure. `index` is gone, if you happen to depend on that. also:

- `Makefile` is finally gone.
- adds a small `recode` utility for demonstrating the usage.