Sorting any permutation of Version objects should always yield the same
result, even if those hold some build metadata.

To that end, the "precedence_key" is now used exclusively for sorting;
direct comparisons between Version objects still ignores the "build"
metadata, using a different precedence key.

For performance improvements, both precedence keys are cached.

Closes: #132

- The README is now a standalone document, also included as an
  "introduction" page;
- A new "guide" section provides more details on most features;
- A couple of typos were fixed.

The main goal was to make the README file perfectly suitable for
rendering on PyPI, while keeping its content available on the standard
documentation on ReadTheDocs.

Including docs and tests.

Closes #98.

Write clear, explicit rules for the `SimpleSpec` syntax.

Their comparison semantics were ill-defined, and mostly an
implementation detail for the old, 'native' specs.

- Use proper theme in development
- Use ReadTheDocs version numbers in titles

This will be used in `sort(..., key=lambda v: v.precedence_key)`.
Remove previous comparison/ordering implementation.

The current implementation relies on a 4-tuple:
- major, minor, patch (as integers) - natural order matches precedence
  rules
- a tuple of identifiers for the prerelease component.

The identifiers for the prerelease components are based on:
- A `NumericIdentifier` class, that compares number using their natural
  order, and always compares lower than non-numeric identifiers
- A `AlphaIdentifier` class for non-numeric identifiers; it compares
  versions using ASCII ordering.
- A `MaxIdentifier` class, that compares higher to any other identifier;
  used to ensure that a non-prerelease version is greater than any of
  its prereleases.

The internal features from those classes will be removed in future
versions:

- The `Spec` class is incompatible with the support of multiple syntaxes
- The `SpecItem` class was an implementation detail, but doesn't support
  complex `Range` combinations.