Feature/Add support for loading webarchive files from manifest

[ianrumac]

What

• Adds manifest : WebArchiveManifest field to Paywall to enable saving paywall as archive
• Adds WebArchiveLibrary and ArchiveCompressor to manage and de/compress MHTML files given a manifest.
• Adds ManifestDownloader to download manifest as File's
• Adds support for non-serializable fetching via current Network layer
• Adds support for non-serializable writing to current Storage layer
• Adds a small DSL for prettier tests with nice crash output
• Refactors WebViewClient into a DefaultWebViewClient and an ArchiveWebClient relying on androix.webkit.WebViewAssetLoader to load local files

How

Downloading

• When a configuration is fetched, it checks for paywalls with manifests and queues them up for a download via WebArchiveLibrary. The library checks if a Manifest is downloaded already or is in Queue, then either skips it or downloads it if necessary.
• The declared files and their necessary reference are downloaded in parallel and compressed into the archive itself.
• Once a paywall and all it's related files (be it from manifest or referenced from the body itself) are downloaded,
• ArchiveCompressor compresses them into an MHTML file and stores them to disk

Viewing

• When a paywall is loaded, depending on the Manifest usage flag it either checks if manifest exists and loads URL if not, or awaits for the manifest to download before loading
• The manifest is decompressed into a set of files and all webview requests are routed to those files through a mock host URL, done by WebArchiveClient overriding the URL requests

Extra

Adds a small Given/When/Then testing DSL to make reading/understanding tests easier