Pick up filter docblock by copying it from the assignment node

If filters are documented like this:

  /**
   * A filter with some exciting documentation
   */
  $foo = apply_filters( 'filter', $foo );

then PHP-Parser associates the docblock with the assignment expression
instead of the function call. This means that the documentation isn't
actually retrieved by the hook reflector.

So, tweak the node visitor to check for assignment expressions that have
docblock and a filter as a subnode. The docblock can then be copied to
the subnode.
More robust hook documentation retrieval strategy.
Taxonomy associations are handled during taxonomy registration
Functions are not hierarchical

Methods (which are stored as functions) have a post_parent, but not of
the same type so we should handle this is a custom fashion (just like
bbPress for example).

Removes `hierarchical => false` as this is the default.
Improve rewrite rules:

 * Remove feeds and static front
 * Plural archive URLs, singular item URLs
Better method URLs: /method/:class/:method/

Method post_names are now namespaced with the method's class. This
allows the method permalink to easily include the class name and it
prevents issues with duplicated method names.
Merge pull request #30 from joncave/improve-cpts

Better method permalinks and simplified CPT registration
Merge pull request #28 from joncave/fix-filter-docs

Pick up filter docblock by copying it from the assignment node