Refile #61 as a Pull Request, add GitHub files, start 0.3

[benlk]

Changes

• Removes 'single' page targeting from post-type archives and from static front pages. PR #72 for #61, thanks to GitHub user dbeniaminov.
• Adds Category targeting on category archive. PR #72 for #61.
• Adds GitHub Pull Request template and Contributing guidelines files.
• Starts the changelog for 0.3: https://github.com/INN/doubleclick-for-wp/milestone/1

Why

This contains @dbeniaminov's fixes from https://github.com/INN/doubleclick-for-wp/issues/61.

Resolves https://github.com/INN/doubleclick-for-wp/issues/61.

Adds the GitHub files because we didn't have them yet.

[benlk]

Testing:

• [x] on single pages, the categoy output contained the categories
• [x] on post-type archive pages, the 'single' term was not output (tested using Link Roundups' rounduplink post_type)
• [x] on single front page, the 'single' term was not output
• [x] on a category archive page, the 'Category' array was properly populated with the category's slug.