Add rewrite_base functionality to rewrites

Previously rewrite_base was a parameter to vhost, but was removed
incorrectly. When rewrites was created to replace rewrite_* parameters,
rewrite_base was forgotten. This adds back the deprecated rewrite_base
parameter and adds the rewrite_base support to the rewrites parameter.
Merge pull request #631 from hunner/fix_rewrite_base

Add rewrite_base functionality to rewrites