refs #1460 - add .setRequestMethod("POST") to piwik.js

Use cases:
 * workaround GET request length limit
 * workaround mod_security or .htaccess rules that block request containing "ht$

Notes:
 * subject to same origin policy



git-svn-id: http://dev.piwik.org/svn/trunk@3231 59fd770c-687e-43c8-a1e3-f5a4ff64c105