bigdecimal documentation

--HG--
branch : trunks
fix exception due to truncated trailing zeros when an integer number is an exact multiple of the bin size

--HG--
branch : trunks
fix that string conversion might return incorrectly 0 after rounding a hidden digit away and leaving a single bin

--HG--
branch : trunks
fix that not enough digit bins were allocated when dividing an integer with a hidden digit

--HG--
branch : trunks
fix another string conversion hidden digit rounding bug

--HG--
branch : trunks
use int64 to store exponent during string conversion because the internal representation works with exponents larger than maxint

--HG--
branch : trunks