 * Added bazaar backend and tests.

 * Removed experimental mercurial backend. I only need "something
   faster than subversion" and Bazaar's worldview fits my brain
   better, so I doubt I'll be maintaining the mercurial backend.
   If you're using it, please complain loudly, because I didn't
   know that you were using it.

 * client.kind(uri) now takes an optional integer `rev` parameter
   like everything else.

 * The strange dictionary returned by .read() now uses keys named
   'body' and 'mimetype' instead of 'body' and 'kind'.

 * Refactored path normalization in SVN backend classes and added
   optional `path_fixer` callable argument to constructor.

   If provided, `path_fixer` should expect to be called with a
   string URI as the only argument. It should return a string URI.
   It can be used to define filesystem layout policies, like
   "always store files prefixed under a date hierarchy" or "add a
   file extension."

   I'm probably overabstracting too early, though, so I won't be
   surprised if I scrap this idea.

 * When writing contents to a file, Sven used to append a newline
   to the contents whether or not the contents already ended with
   a newline. Now, sven will only append a newline if the content
   doesn't already end in a newline.

 * Implemented custom resource properties for bazaar backend,
   because it doesn't have any concept like `svn propget/propset`.

   Resource properties are implemented as independently versioned
   files. File and directory properties live in separate locations,
   because that seemed easier and more logical somehow.

   For a file '/foo/bar/baz.txt' its 'mimetype' property lives at
   '/.sven-meta/.mimetype/foo/bar/baz.txt' and its 'fleem' property
   lives at '/.sven-meta/.fleem/foo/bar/baz.txt'.

   For a directory '/foo/bar/' its 'mimetype' property lives at
   '/foo/bar/.sven-meta/.mimetype' and its 'fleem' property lives
   at '/foo/bar/.sven-meta/.fleem'.

   Note that for normal files read and written directly by the user,
   sven's default behavior appends a newline to the end of the file
   if there is none. For these metadata files, sven does not append
   a newline to the end of the file.
