Added integer validation

Closes vlucas#118

README.md

@@ -175,6 +175,20 @@ If the environment variable is empty, you'd get an Exception:
 One or more environment variables failed assertions: DATABASE_DSN is empty
 ```
 
+### Integer Variables
+
+You might also need to ensure the the variable is of an integer value. You may do the following:
+
+```php
+$dotenv->required('FOO')->isInteger();
+```
+
+If the environment variable is not an integer, you'd get an Exception:
+
+```
+One or more environment variables failed assertions: FOO is not an integer
+```
+
 ### Allowed Values
 
 It is also possible to define a set of values that your environment variable

src/Validator.php

@@ -62,6 +62,21 @@ function ($value) {
         );
     }
 
+    /**
+     * Assert that each specified variable is a number.
+     *
+     * @return \Dotenv\Validator
+     */
+    public function isInteger()
+    {
+        return $this->assertCallback(
+            function ($value) {
+                return ctype_digit($value);
+            },
+            'is not an integer'
+        );
+    }
+
     /**
      * Assert that each variable is amongst the given choices.
      *