Patch 6523 by Steven:

- Strips the leading / from URLs to match with block paths. The reason is that everywhere else in Drupal, we use URLs without leading slash (e.g. URL aliasing). For consistency we should keep this behaviour everywhere. The replacement uses ereg_replace instead of str_replace, but the expression now only match the beginning of the URI so in fact it's faster, esp. for longer URIs.

- Updates the block docs about path matching: gets rid of leading slash, adds initial matching operator '^' and emphasises the slash behaviour (which was not mentioned anywhere before)

- Replaces the example expression for a negative match. It was horribly broken and odd, the replacement in this patch is correct and simpler.