Standardised function signatures / names for encoding functions
Removed unneeded lines from read_base64_block()

These lines:

	out[1] = '\0';
	out[2] = '\0';
	out[3] = '\0';

... are remnants from before the conversion function was migrated to work with
a bytestream. The BS doesn't need null terminating, and so it's not necessary
to write these values in.

closes #5
Changed bs_encode_size to return BSresult rather than size_t

The functions for returning encoded sizes return a size_t rather than a
BSresult. This now no longer makes sense - we should return BS_BAD_ENCODING if
the encoding isn't known.

fixes #16