Add initial basic tests for download option

tests/test_download.py

+from os import makedirs
+from os.path import join
+import textwrap
+from test_pip import here, reset_env, run_pip, pyversion, lib_py, get_env, diff_states, write_file
+
+def test_download_if_requested():
+    """
+    It should download and not install if requested.
+    
+    """
+    reset_env()
+    result = run_pip('install', 'INITools==0.1', '-d', '.', expect_error=True)
+    assert 'INITools-0.1.tar.gz' in result.files_created
+    assert join(lib_py + 'site-packages', 'initools') not in result.files_created
+
+def test_single_download_from_requirements_file():
+    """
+    It should support download from PyPi from a requirements file"
+    """
+    reset_env()
+    write_file('test-req.txt', textwrap.dedent("""
+        INITools==0.1
+        """))
+    result = run_pip('install', '-r', 'test-req.txt', '-d', '.', expect_error=True)
+    assert 'INITools-0.1.tar.gz' in result.files_created
+    assert join(lib_py + 'site-packages', 'initools') not in result.files_created