remove use of IConvertible so it works with portable libraries.

[prabirshrestha]

Remove the #if part and always use the way it is done in NETFX_CORE.

#if NETFX_CORE
 type == typeof(int) || type == typeof(long) || type == typeof(double) || type == typeof(float) || type == typeof(bool) || type == typeof(decimal) || type == typeof(byte) || type == typeof(short)
#else
 typeof(IConvertible).IsAssignableFrom(type)
#endif

Seems like portable library update broke something again or it breaks when targeting wp8, metro and net 4.5 as PCL.

[prabirshrestha]

fixed in v0.26.0