_get_plugin_basename_from_slug should check for a '/'

This addresses #147 (https://github.com/thomasgriffin/TGM-Plugin-Activation/issues/147)
Merge pull request #154 from jtsternberg/master

_get_plugin_basename_from_slug should check for a '/'
Merging branches.
v2.4.0 - here we go. Fixes a ton of bugs and other issues.
Say hello to TGMPA 2.4.0!
Readme formatting.
More readme formatting.
Cmon Markdown, be nice to me!
Fixes #171.

Fixes permissions error by using network_admin_url instead of
admin_url. Reverted back to admin_url since after testing it works fine
for MultiSite.
Added textdomain in `_n_noop` function
Merge pull request #173 from KingYes/master

Added textdomain in `_n_noop` function
Fix: spelling
Fixes #174.
Fixes #180. Fixes #182. Fixes #183.
Merge pull request #179 from tivnet/master

Fix: spelling
Fixes #178.
Fixes #176.
Fixes #185.
Add filters to enable more customization

* Added `tgmpa_submenu_parent_slug` filter - modify what menu the sub-page is under, instead of requiring it under "Appearance". This allows the page to be added to a plugin's sub-menu.
* Added `tgmpa_table_columns` filter - modify table columns in table view
* Added `tgmpa_table_data_item` filter - modify plugin data that's ready for the table output
* Added `tgmpa_notice_rendered_action_links` filter - modify final action link output

I wanted to add a "Description" column that says why should an user install this plugin.

```
array(
    'name'      => 'WordPress SEO by Yoast',
    'slug'      => 'wordpress-seo',
    'description' => 'This is a valuable SEO plugin.',
);
```

These filters allow much more flexibility in the setup.
Removed hard-coded "description" setting