Added paInt16, paInt8, format using new "pa_common/pa_convert.c" file.

Added paInt16, paInt8, format using new "pa_common/pa_convert.c" file.
Return error if opened in mono mode cuz not supported.
Add support for Pa_GetCPULoad();
Fixed timestamp in callback and Pa_StreamTime() (Thanks n++k for the advice!)
Check for invalid sample rates and return an error.
Check for getenv("PA_MIN_LATEWNCY_MSEC") to set latency externally.
Better error checking for invalid channel counts and invalid devices.