Add 'sh' script DSL method - close #142

build_tests/sh/sh.rb

@@ -0,0 +1,4 @@
+build do
+  sh "echo", "hi  there"
+  sh(["echo  1  2"])
+end

build_tests/sh/sh_fail.rb

@@ -0,0 +1,4 @@
+build do
+  sh "foobar42"
+  sh "echo", "continued"
+end

build_tests/sh/sh_fail_continue.rb

@@ -0,0 +1,4 @@
+build do
+  sh "foobar42", continue: true
+  sh "echo", "continued"
+end