fix(lib): enums are now namespaced + refactor

See rust-lang/rust#18973

- The `linspace` and `logspace` functions have been moved into their own crate
- The `color` module has been removed, the `Color` enum is now in the root of
  the crate
- The following enums have been moved into the root of the crate: `Axis`,
  `Grid` and `Scale`
- Renames:
  - `HorizontalPosition` -> `Horizontal`
  - `LeftJustified/RightJustified` -> `Justification::{Left, Right}`
  - `Vertical::Middle` -> `Vertical::Center`
  - `VerticalPosition` -> `Vertical`

[breaking-change]
Merge pull request #11 from japaric/fix

fix(lib): enums are now namespaced + refactor