Replace dev chain special case with preset

Add --min-gas-price (does the same as legacy --gasprice) and tweak
gas_pricer_config() and test cases.  Dev preset will still be
overridden by CLI arguments, including --gasprice.

parity/cli/presets/config.dev.toml

@@ -0,0 +1,6 @@
+[parity]
+chain = "dev"
+
+[mining]
+reseal_min_period = 0
+min_gas_price = "0"

parity/cli/presets/mod.rs

@@ -16,8 +16,7 @@
 
 pub fn preset_config_string(arg: &str) -> Result<&'static str, &str> {
     match arg.to_lowercase().as_ref() {
-        "poa" => Ok(include_str!("./config.poa.toml")),
-        "pow" => Ok(include_str!("./config.pow.toml")),
+        "dev" => Ok(include_str!("./config.dev.toml")),
         _ => Err(arg.clone())
     }
 }