Add PATH manipulation methods - close #126

build_tests/simple/path_append/foobar

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "foobar!"

build_tests/simple/path_append/gcc

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+exit 42

build_tests/simple/path_prepend/flex

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "flex!"

build_tests/simple/pathing.rb

@@ -0,0 +1,10 @@
+path_prepend "path_prepend"
+path_append "path_append"
+
+build do
+  Environment.new do |env|
+    system("flex")
+    system("foobar")
+    env.Object("simple.o", "simple.c")
+  end
+end