slog-rs logging (#171)

* added global slog instance, changed all logging macro formats to include logger instance
* adding configuration to logging, allowing for multiple log outputs
* updates to test, changes to build docs
* rustfmt
* moving logging functions into util crate

api/src/lib.rs

@@ -21,7 +21,7 @@ extern crate secp256k1zkp as secp;
 
 extern crate hyper;
 #[macro_use]
-extern crate log;
+extern crate slog;
 extern crate iron;
 extern crate router;
 extern crate serde;