Adding a way to disable all proxy processing (#691)

* Adding a way to disable all proxy processing

When the proxy field in configuration is === false all proxy processing is
disabled. This specifically disable the 'http_proxy' environment variable
handling.

Fixes #635
Related to #434

* Change readme wording

From review comment on PR (#691)

test/unit/adapters/http.js

@@ -311,6 +311,23 @@ module.exports = {
     });
   },
 
+  testHTTPProxyDisabled: function(test) {
+    // set the env variable
+    process.env.http_proxy = 'http://does-not-exists.example.com:4242/';
+
+    server = http.createServer(function(req, res) {
+      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
+      res.end('123456789');
+    }).listen(4444, function() {
+      axios.get('http://localhost:4444/', {
+          proxy: false
+        }).then(function(res) {
+          test.equal(res.data, '123456789', 'should not pass through proxy');
+          test.done();
+        });
+    });
+  },
+
   testHTTPProxyEnv: function(test) {
     server = http.createServer(function(req, res) {
       res.setHeader('Content-Type', 'text/html; charset=UTF-8');