API sweetening, example PHP, small fixes

• Added API method save($filename).
• Added image dir source to create().
• create() now returns $this for method chaining.
• Fixed errors in reset(). (Wow, even there?!...)
• Added a slideshow example script (index.php) for convenience.

Better delay times & error handling

• Delay values can be selectively omitted (the previous one will by used by default).
• Changed default delay time to 100ms.
• Fixed delay time units (from ms to 0.01s).
• API change: $->gifBinary() -> $->gif().
• Almost real error handling for image loading...
• Various other code improvements (errors, control flow, cleanup etc.)
• Some cleanup & refresing of the docs. (incl. comments)

Stylistic refreshments, small fixes & improvements everywhere

TODO:

• Default anim delay doesn't seem to be 40 ms, at all. :-o

DONE:

• Fix ERR01 "Source is not a GIF image.": there's a .png in the examples!
  -> And it does support non-GIF files actually.
  Moved the error check to resource inputs only, and changed it to
  "Resource is not a GIF image.".
• create() should iterate $frames with foreach() not for assuming direct
  indexes from 0 to < count.
  (The array keys can be anything, and should not affect the results.)
• Removed unused $mode from reporting ERR02.
• $duration now has a meaningful default in create().
• $frames was incorrectly made an optional arg. of create().
• Support custom even timing without an array.
• Separate ERR00 from $durations not being an array.
• Fix leftover $GIF_tim in create().
• Renamed method getGif() to get().
• Renamed class to AnimGif (from GifCreator).
• Made $this->version a define (VERSION).
• Made $this->errors really static (self::$errors).
• Moved encodeAsciiToChar() out from the class to the namespace root as a general utility fn.
• Moved getGif to the "public" section (and removed the Getter/Setter section).
• Moved reset() closer up to the ctor.
• Changed comments here & there.
• Whitespaces: fixed some tab/space mismatch etc.
• Changed {$i} indexes to [$i] in gifBlockCompare(), just for fun. ;-p