main: Set trigger before enabling the buffer

As advised in:
http://article.gmane.org/gmane.linux.kernel.iio/14742
main: Scale readings so they're normalised

By applying the scale to the accelerometer, we should be able to
handle accelerometers that use more than 1G as the max reading.

Closes: #4
main: Fix compilation warning

io-sensor-proxy.c: In function ‘process_scan’:
iio-sensor-proxy.c:790:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘ssize_t’ [-Wformat=]
    g_debug ("Not enough data to read (read_size: %d scan_size: %d)", data.read_size, (int) or_data->scan_size);
    ^