[CHORE] add IEEE-754 Double Precision float support

for an 8 byte representation of floats
[FEATURE] human readable demurrage support

- human readable string to Currency
- Currency to human readable string
[TEST] test human readable Currency parsing
[TEST] update amount test for interest bearing currencies
[FIX] Currency by number default type

should be a non-interest bearing currency
[FIX] parse_number should create a non-native currency

"1" just means "some issuer, don't care which"
[FIX] parse_number correctly setting _is_negative
[TEST] add more from_number tests
[FEATURE] allow numbers, hyphens and spaces in full currency
[FEATURE] support full_name in to_human() and to_json()
Merge pull request #83 from geertweening/feature-demurrage

Interest bearing currency parsing support