mg_read() does not block on content_len=0. mg_read() reads until socket is closed if content-length is not provided

Fix for the SCRIPT_NAME in the presence of PATH_INFO, https://code.google.com/p/mongoose/issues/detail?id=403

Always use absolute path for document_root. If relative path is used, it is relative to mongoose.exe directory. See https://github.com/valenok/mongoose/issues/181

Lua functionality change: print renamed to mg.write, read renamed to mg.read. Expanded user manual, fixed example pages.