use Perl

Sure, Mech's been around for over a year, but it's now officially 1.00. I figure it's about time we hit 1.00, and this version seems like a good place to do it, because of the potential breakage described below...

[THINGS THAT WILL BREAK YOUR CODE]
* Header handling has changed.  There is no more package variable
  %headers that holds all the headers to be added.  They are now
  added on a per-object basis.

  If you were adding a header with add_header(), and the code
  relied on that header still being set later on in a later
  instance of the variable, that code will now break, because
  the later instance won't have the header set.

[ENHANCEMENTS]
* You can now prevent a header from being sent by adding it with
  an undef value, as in:

    $mech->add_header( Referer => undef );

[FIXES]
* Now correctly adds Accept-Encoding to all requests that need it.

[INTERNALS]
* Added new $mech->_modify_request($req) method to do all the
  HTTP header modification before the actual request gets
  sent off.  Subclasses are able to override it if they want.

* Removed the unused Compress::Zlib stuff.