Fixed binary_prefix handling, renamed it to si_prefix

Previously there was the following when binary_prefix was False:
    div = 1024
    unit = kB
(unit is inconsistent with div).
Now it is the following when si_prefix is False:
    div = 1024
    unit = KiB
(according to https://en.wikipedia.org/wiki/Binary_prefix IEC prefix kibi is 
abbreviated as Ki, not ki (note the case)). This means that in segments prefixes 
has changed, but not displayed values.
Added powerline.lib tests