Add wasm32-unknown-unknown compile target

Currently we cannot compile to web assembly due to downstream dependencies still using deprecated crates.

- [x] [Bump bind-gen to version that no longer uses rustc-serialize for rocksdb fork](
https://github.com/ETCDEVTeam/emerald-rocksdb/commit/bf1f97cc9c3f1eae7f7b2d9850c8c406747a6e47)

- [x] #285
- [x] rust-num/num#366 -> Duplicate of -> rust-num/num#357
- [x] r8d8/rust-scrypt#4
- [ ] DaGenix/rust-crypto#437 -- fork available - Using less than optimal fork in the current PR. Take a look at this issue, seems like https://github.com/RustCrypto is the better way to go here.
- [ ] rust-bitcoin/rust-bitcoin#96 -- can we remove this? alternate mnemonic implementation?

Once all the above are resolved, we should be good to go for wasm targeting.