Added getUser(String username) and test for same.

src/main/java/org/gitlab4j/api/UserApi.java

@@ -74,6 +74,24 @@ public class UserApi extends AbstractApi {
         return (response.readEntity(User.class));
     }
 
+    /**
+     * Lookup a user by username.
+     *
+     * NOTE: This is for admin users only.
+     *
+     * GET /users?username=:username
+     *
+     * @param username the username of the user to get
+     * @return the User instance for the specified username
+     * @throws GitLabApiException if any exception occurs
+     */
+    public User getUser(String username) throws GitLabApiException {
+        GitLabApiForm formData = new GitLabApiForm().withParam("username", username, true);
+        Response response = get(Response.Status.OK, formData.asMap(), "users");
+        List<User> users = response.readEntity(new GenericType<List<User>>() {});
+        return (users.isEmpty() ? null : users.get(0));
+    }
+
     /**
      * Search users by Email or username
      *

src/test/java/org/gitlab4j/api/TestUserApi.java

@@ -82,4 +82,11 @@ public class TestUserApi {
         assertNotNull(currentUser);
         assertEquals(TEST_USERNAME, currentUser.getUsername());
     }
+
+    @Test
+    public void testLookupUser() throws GitLabApiException {
+        User user = gitLabApi.getUserApi().getUser(TEST_USERNAME);
+        assertNotNull(user);
+        assertEquals(TEST_USERNAME, user.getUsername());
+    }
 }