preserve leading zeroes when exporting private key to hex/wif

otherwise keys starting with a zero would end up having a
different address when imported somewhere else.
separated test for leading zero
Fixed bug with leading 0s in privkey hex format

Support#int_to_hex now takes an optional second argument specifying
the required number of characters in the output string.
Where known to matter, the uses of int_to_hex have been modified
to specify the required chars.
bump version for release