Allow to change nonce on contract before sending transaction

Reason: It allows a developer to calculate transaction nonce on his own.
May be needed when you want to control transactions order or calculate
nonce using transaction pool rather than relying on current client
algorithm.

Relates to:
- EthWorks#35
- ethereum/go-ethereum#2880

lib/ethereum/contract.rb

@@ -5,7 +5,7 @@ class Contract
 
     attr_reader :address
     attr_accessor :key
-    attr_accessor :gas_limit, :gas_price
+    attr_accessor :gas_limit, :gas_price, :nonce
     attr_accessor :code, :name, :abi, :class_object, :sender, :deployment, :client
     attr_accessor :events, :functions, :constructor_inputs
     attr_accessor :call_raw_proxy, :call_proxy, :transact_proxy, :transact_and_wait_proxy
@@ -247,7 +247,7 @@ def build
         extend Forwardable
         def_delegators :parent, :deploy_payload, :deploy_args, :call_payload, :call_args
         def_delegators :parent, :signed_deploy, :key, :key=
-        def_delegators :parent, :gas_limit, :gas_price, :gas_limit=, :gas_price=
+        def_delegators :parent, :gas_limit, :gas_price, :gas_limit=, :gas_price=, :nonce, :nonce=
         def_delegators :parent, :abi, :deployment, :events
         def_delegators :parent, :estimate, :deploy, :deploy_and_wait
         def_delegators :parent, :address, :address=, :sender, :sender=