v3.0.0: Took long enough, to be honest

BREAKING:
  * Most functions dealing with discovering/resolving registry repositories,
    notably:
    + renamed ops::MainRepoPackage                -> RegistryPackage
    + renamed ops::installed_main_repo_packages() -> installed_registry_packages()
    + ops::intersect_packages()' to_update tuple to include desired registry
    + ops::get_index_path() actually hashes and resolves the correct name,
      instead of finding the latest directory
    + ops::get_index_url() takes the registry name/URL to resolve and returns both
      the URL and the Cargo name
  * CLI should be fully compatible

New:
  * Support for arbitrary registries, as opposed to just crates,io
    (with installation via "(registry):pkgname" and out-of-box updates)
    (263b2c2c4b6218da31db186d0d739453a437e420)
    (30b6398afd41abf1afa031c571bef438a4bd694e)
    (5cbd4edbd87670477574d36f9ece619c8be5da02)
    (d767162a59f9a87d328930e1f27d819c5dc4231c)
    (a369fa77dbfa5f1c28b60d0f0cfca298a6000b91)
    (1b27f6e43940f49551a730e427b1762456e20e8c)
    (#128)
  * ops::registry_shortname() for hashing registry URLs into directory names
    (263b2c2c4b6218da31db186d0d739453a437e420)
    (30b6398afd41abf1afa031c571bef438a4bd694e)

Internal:
  * Don't install CMake anymore. Not sure why it was in the first place
    (bc507bdabd3638b5c62f40e9783582c764ac01ca)
  * Correctly install Clippy on Travis
    (fe8edbef506ce96c64e1881f0f91e2bd92ebb100)
  * Package both executables on AppVeyor
    (fe8edbef506ce96c64e1881f0f91e2bd92ebb100)
    (2321e2d6fc80deaa74766f87e0f5cf0f336288ce)
  * Fix some clippy warnings
    (b23efa1a0179da92e65e4dd431a59386a318d144)
  * Fixed typos in tests
    (1a0ea8d6e685e9851e4f66ad8fde191f7aac98e3)