Use std::stringstream instead of snprintf() for double->string conversion

snprintf() will use the current locale for converting a double to a string,
which will use a ',' instead of a '.' in some locales (e.g. de_DE).
std::stringstream allows setting the locale to "C" to always get a '.'.
Merge pull request #2 from steffen-kiess/fix-locale

Use std::stringstream instead of snprintf() for double->string conversio...