Plugin ready for testing.

- Correct typos on get_term() calls caused by search and replace.
- register activation hook
- add function to create term taxonomy meta table
- add plugin header
add index file to avoid listing directory contents
Correct main plugin file name.

* renamed slimline-term-taxonomy.php to slimline-term-taxonomy-meta.php.php
* corrected add_metadata() calls to correctly pass $unique instead of non-existant $prev_value