Flexible 'interpret' function about bitstring HOT 2 CLOSED

On reflection, I'm not sure I like the '10:' bit-moving token fragment. I'm in 
two
minds about the '+5' as well.

Another point is that 'rest' is redundant. As it stands 'bits' will accomplish
exactly the same (a BitString that takes up the rest of available bits).  

Rather than 'interpret', a better name is 'unpack'. This then implies a 'pack'
function that can be used to create new BitStrings.

>>> format = '0x6422, uint4, uint4, 0b1, ue' # has three 'placeholders'
>>> s = bitstring.pack(format, 3, 2, 105)

Done for version 0.5.1