[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 #86 from shekenahglory/develop

Identity functions for blob vault storage
[BUG] merge IE11 signature fix
Merge remote-tracking branch 'upstream/develop' into develop
Merge pull request #83 from geertweening/feature-demurrage

Interest bearing currency parsing support
[FEATURE] support numeric currency codes

ISO 4217 states support for both alphabetic and numeric codes. rippled adheres to the ISO 4217 as stated in the Currency_Format on the wiki (https://ripple.com/wiki/Currency_Format) and there are trustlines out there with numeric currency codes.

The three-digit numeric code is useful when currency codes need to be understood in countries that do not use Latin scripts and for computerised systems. Where possible the 3 digit numeric code is the same as the numeric country code.
Merge pull request #87 from geertweening/feature/numeric_currency_code

[FEATURE] support numeric currency codes
Merge remote-tracking branch 'upstream/develop' into develop
[CHORE] Move HTTP request signing functionality into its own class.
[FEATURE] Add ability to calculate ledger entry keys.
Merge pull request #88 from justmoon/feature/generate_ledger_entry_indexes

Generate ledger entry indexes