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

pool/Cargo.toml

@@ -7,7 +7,7 @@ authors = ["Grin Authors <mimblewimble@lists.launchpad.net>"]
 grin_core = { path = "../core" }
 grin_store = { path = "../store" }
 grin_p2p = { path = "../p2p" }
-secp256k1zkp = { path = "../secp256k1zkp" }
+secp256k1zkp = { git = "https://github.com/mimblewimble/rust-secp256k1-zkp" }
 time = "^0.1"
 rand = "0.3"
 log = "0.3"