The Specpocalypse: Reorganize spec structure.

This is large, complicated, and requires some explanation.

 * Rubyspec updated to master, now in `spec/corelib`
 * stdlib tests that were removed from rubyspec now reference their counterpart
   repository in rubysl.  These live in `spec/stdlib`
 * Filters have been moved from `spec/opal/filters` to `spec/filters`, as they apply
   to rubyspec and rubysl specs.
 * The base directory in the rake tasks have changed from `spec/opal` to `spec`, therefore
   a few paths had to be modified, namely ERB template paths.
 * The `rubyspecs` file now lives in `spec`, and have to specify paths relative to itself.
   This file specifies files in either `corelib` or `stdlib`
 * `rubyspecs` has been edited to omit tests that Opal was previously able to parse, but
    can no longer.  These are only omissions related to `SyntaxError`.  An unfortunate
    amount of coverage has been lost here.
 * `spec/filters/uncategorized` has been created.  These contain specs that now fail due to
    upgrading rubyspec, or new specs in rubyspec that fail.  Some of these are re-wordings of
    specs we pass.  No attempt to match these strings with existing filter strings has been made:
    this will be a project onto itself.  No editorializing has been attempted, except to
    allow the Time specs to fail, which have never passed on my machine.  The intent is, these
    entries  will dwindle in the future as bugs are fixed, or they are explicitly moved to `bugs`
    or `unsupported`.