0.4.0

• Add the chroot tricks (#23 and #25) to README.md and added examples.
  • Fix backtracking that occurs when using guard and chroot.
  • Fix bug where the same tag may appear in the result set multiple times.
• Performance optimizations when using the (//) operator.
• Make Scraper an instance of MonadFail. Practically this means that failed
  pattern matches in <- expressions within a do block will evaluate to mzero
  instead of throwing an error and bringing down the entire script.
• Pluralized scrapers will now return the empty list instead mzero when there
  are no matches.
• Add the position scraper which provides the index of the current sub-tree
  within the context of a chroots's do-block.