Set BigNumber to not use exponents

This makes `BigNumber` output the full plain number (with no exponents). Taken from http://mikemcl.github.io/bignumber.js/#exponential-at

`1844674407370955e4` is now  `18446744073709550000`
`5.4210108624e-10` is now `0.00000000054210108624`

https://github.com/stellar/stellar-client/issues/904