SEC clean and bigi upgrade

Chained on #184.

This pull request finishes a `TODO` in `sec.js` to use a hash map instead of successive if statements for the different curves, performs a cleanup of the code formatting and finally upgrades the `bigi` version to 1.1.0. 

Comparison link: https://github.com/dcousens/bitcoinjs-lib/compare/ecclean...secclean