Removed setlocale instances https://github.com/oetiker/rrdtool-1.x/issues/504

This branch does the following:
1) Removes instances of setlocale
2) Added rrd_strtod function, which is a locale independent version of strtod function 
3) Made most of the code to use rrd_strtod & rrd_snprintf instead of strtod, atof & snprintf
is this really necessary ? it would seem that librrd_th should already contain rrd_strtod