Fix undefined method error on activation

Call to undefined method WP_Simple_Share_Buttons::init()
Fix google plus variable typo
Version bump, remove dead code, no loose comparisons
Refactor API + more

- Refactor API
- Filterable post titles and authors
- Output conditionals and post types are selectable
Merge pull request #2 from khromov/refactor-api

API improvements, refactoring