Release 0.9.4

This release corresponds to release 0.9.4 of jmcnamara/libxlsxwriter.

Note: To get a locale independent version of the library, on compiling one should always define the symbol USE_DOUBLE_FUNCTION.

While the original library uses the system function snprintf (which is locale dependent) to format floating point values, and replaces the decimal separator afterwards, if symbol USE_DOUBLE_FUNCTION is defined, this library variant uses a very fast conversion function, based on the code of emyg_dtoa, which is always locale independent.