Pyramid-frontend

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- Switches to a class-based specification for theme assets. This enables easy
customization and addition of asset handlers (inside our outside of the
pyramid_frontend codebase), and makes it easier to configure asset-specific
settings (e.g. version of lessc executable to use) on the particular asset,
rather than theme-wide.
- Adds an SVG asset handler, so that SVG files can be tracked in a way similar
to .less, .js, etc.

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- Allow themes to specify an alternate lessc executable path. This is necessary
specifically to allow some themes to use Bootstrap 2 and some themes to use
Bootstrap 3: the two versions use mutually incomaptible versions of the less
compiler.

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- Added looking up the URI of a template based on filename, and then using that
URI to refer to a template that may not be at the end of an inheritance
chain.
- Fix a bug which prevent using inherited namespaces from an inherited
template.
- Clean up some Python 3 compatibility mechanisms that may have caused issues
on certain platforms.

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- Python 3 compatibility.
- Adopted ``tox`` for testing and static analysis.
- Some documentation improvements.

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- Initial release.