fix issue #1; oddly Chrome barfs in the JS code due to the malformatted HTML DOCTYPE. This removes the error when browsing to example.html
set up a few README dummy files so git will create the upload directory tree on git clone and then `example.html` can be tested out of the box.
- added sanitizeFilename() function to clean user-provided (browser-provided, rather, but you cannot trust the sender = 'browser') metadata such as filenames (what's stopping a user from speccing a 'filename' such as `../../../../../../../../etc/passwd` otherwise?)

- class BigUpload now always returns response arrays instead of JSON; user code (here shown at the bottom of the PHP file) can encode this as JSON as pass it on to the browser. This change allows other PHP to use the BigUpload class and augment/tweak the responses before sending them off to the browser (think embedding BigUpload in PHP frameworks / routes).

- class BigUpload now returns a non-zero HTTP response code as part of the response array if any error occurred. Note that 0(zero) still is the signal for 'all okay'; see the 'user code' at the bottom of the PHP file how this turned very quickly in a proper-always-HTTP-response coded response.

- consistent behaviour under all circumstances: let PHP produce the correct MIME type and always respond in JSON format, even when the BigUpload and/or user code crashes and throws an exception for whatever reason.

- BigUpload::postUnsupported() now returns okay/error array results like the other BigUpload methods.

- BigUpload is now fully parameterized, i.e. it does not assume metadata to exist in specific (hardcoded) GET or POST parameters (e.g. `POST[name]`)

- Added a few sanity checks and a more robustness to the PHP example code.

- When invoking the PHP 'page' directly from the browser (rather than from example.html and the JavaScript code therein, the PHP page will output a reasonably legible response (even when it's JSON shown on-screen in yuor browser)

NOTE: there's still ways to screw around with this thing, e.g. hostile takeover/corruption of others' uploads by 'stealing' (guessing) their upload tempname. To be fixed in a later commit.

Tested on WAMP 2.5 + Chrome/Win8-64 (Stable); single files of 900+MB upload fine (and binary comparison with their original show the uploads are fine).