Asset collections

This adds the ability to add multiple asset collections to compass. For each collection there is basically a set of asset options like you would use by default. This effectively forms a path list of lookup locations for images and fonts and sass files. The sass files in the project form a "default" collection of assets which allows the asset resolution code to be homogenous. URL construction and resolution is greatly simplified and dried up compared to before.

There are a couple of small backwards incompatibilities introduced in this patch. I'm trying to decide if this is worth a major version bump or if I should go through a deprecation phase first:

1. `http_path = :relative` is removed in favor of the long time preferred option `relative_assets = true`
2. The url passed to `asset_host` and `asset_cache_buster` now includes the full root-relative path (instead of being relative to the asset's http directory). This allows for better disambiguation and let's the relative path operation work correctly even when asset_cache_buster completely changes the url of the asset.
3. Images that are missing are now always an error instead of only sometimes being an error depending on configuration.

This is still a WIP, but I wanted to get code review started.

Documentation and sass file support is still pending.
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