Reverse button works!

Created reverse.php and got communication between javascript --> PHP
--> SoX to reverse a file and reload the waveform. Had to use $.trim()
and escape() to remove whitespace that gets added in php / ajax.

Next steps:
* add more effects using SoX.
* figure out how to determine the position of Green and Red markers so
that SoX can make edits.
* figure out how to loop playback in webaudio.
* figure out how to handle Undo sessions (will an array of filename
strings do the trick? need a text file? a database?)
* add Undo, Redo, Download buttons.
* CSS: Why doesn't "data-action" work for items in dropdown menus, only
buttons? Gotta get this working for dropdowns.
* Add canvas to visualize audio playback.
* When files are uploaded using SoX, encode as PCM for faster edits,
but store original extension, and reconvert on download.
* Add "Zoom" with an a full waveform up top.
* Set loop points and zoom window by dragging.