Replace secp256k1 directory with repository dependency (#126)

* workaround the "commitment is not a public key" problem by using to_two_keys() from rust-secp256k1-zkp
* remove secp256k1zkp, update the various Cargo.toml to use rust-secp256k1-zkp from external git repo, update code to not rely on to_pubkey (and use verify_from_commit)
* secp256k1zkp from git
* cleanup secp256k1 dependencies
* gitignore node1 and node2 dirs

api/Cargo.toml

@@ -10,8 +10,7 @@ grin_chain = { path = "../chain" }
 grin_pool = { path = "../pool" }
 grin_store = { path = "../store" }
 grin_util = { path = "../util" }
-secp256k1zkp = { path = "../secp256k1zkp" }
-
+secp256k1zkp = { git = "https://github.com/mimblewimble/rust-secp256k1-zkp" }
 hyper = "~0.10.6"
 iron = "~0.5.1"
 log = "~0.3"