feedback from Wordpress VIP review

Changes required for the wordpress vip program. Feedback is below.

```
Thanks for submitting your plugin for review. I've reviewed the code and have the following questions/notes below.

Issues:

please replace all i18n functions with escaped equivalents. For example, us eschtml_e() etc. in place of _e(), and esc_html() etc. in place of _().

class-gumroad.php

line 76: consider removing this an the associated function. Most VIPs will load your plugin file in code and may never see an activation message.

line 85: not required on VIP. You can omit this action and the check_wp_version function

line 118: this can be simplified ... just call load_plugin_textdomain( 'gum', false, $gum_lang_dir ) Is there a reason that won't suffice?

uninstall.php Remove this file if you'd like since it will not execute on wordpress.com

includes/hooks-examples.php test. Please remove this file..

includes/misc-functions.php:

line 18: please add escaping as you build the overlay button. Use esc_html(), esc_attr(), esc_url() etc.

gum_get_pib_downloads and gum_is_pib_active aren't used and should be removed.
```