Update examples to use the native token contract

[sisuresh]

#62 updated the liquidity pool contract to use the native token contract, but this was reverted in #76 to make authorization changes easier (the native token contract doesn't have access to the auth code, so we'd have to copy it in).

Once the native token contract can do authorization, the examples should be updated to use the native token contract.

[sisuresh]

Closed by #128.