dont reload when just clicking the dropdown, must select something

fixes #19
add reload-button. closes #19.
update readme
use wordpress built in class instead of own
Don't go before page 0 or after total pages. Could happend if you navigate quickly with keyboard.
try to fix wp deprecated notice in latest beta

wp beta says wpdb::escape is deprecated and that I shall use esc_sql instead. however esc_sql uses wpdb::escape, resulting in the same notice again. anyway, i try to do as they say :)
Setup includes folder and move files

New folders includes/assets, includes/classes and includes/images
Introduce do-not-modify-index.php

And move main index.php contents to simple-history.php
Cleanup plugin header

Move license declaration to license.txt
Introduce singleton pattern

Rename main class to Simple_History
Move defines and globals into singleton

deprecate SIMPLE_HISTORY_VERSION, SIMPLE_HISTORY_NAME and
SIMPLE_HISTORY_URL
Separate plugin functionality

Move functions to functions.php, admin.php and settings.php
Refactor loading textdomain

Introduces `load_textdomain` method
Move all admin related methods to includes/admin.php
Cleanup Simple_History_Admin class

New Simple_History_Admin::register_admin_settings() and renamed
Simple_History_Admin::wp_dashboard_setup() to
Simple_History_Admin::dashboard_widget_history()