Add template for issues and PRs

Closes gh-551.

.github/CONTRIBUTING.md

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+Issues
+------
+
+If you've found a bug or have a great idea for a new feature that will help **all users** of impress.js, let us know by [adding your suggestion]
+(https://github.com/bartaz/impress.js/issues/new) to the [issue tracker](https://github.com/bartaz/impress.js/issues).
+
+Guidelines:
+
+* If reporting a bug, please provide a [simplified example](https://sscce.org/) on [Pastebin](https://pastebin.com/) or [JsFiddle](https://jsfiddle.net/).
+
+Pull Requests
+-------------
+
+[Pull Requests](https://help.github.com/articles/using-pull-requests/) should be opened against the [master branch]
+(https://github.com/bartaz/impress.js/tree/master). But remember that the team will only accept code that fits the purpose of impress.js.
+
+Guidelines:
+
+* If proposing a feature, make sure to discuss that as an issue first.
+* Create a new [topic branch](https://github.com/dchelimsky/rspec/wiki/Topic-Branches) for every separate change you make.
+* Make sure impress.js runs successfully on as many browsers as you can test.
+* Run `npm lint` to make sure the code is consistent with the project standards.

.github/ISSUE_TEMPLATE.md

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+**Make sure these boxes are checked before submitting your issue, thank you!**
+
+- [ ] I have read and complied with the [Issues Contributing Guidelines](https://github.com/impress/impress.js/blob/master/CONTRIBUTING.md#issues).
+- [ ] If am reporting a bug, I have a [simplified example](https://sscce.org/) on [Pastebin](https://pastebin.com/) or [JsFiddle](https://jsfiddle.net/) that reproduces the issue.
+- [ ] I have used the [Search Tool](https://github.com/impress/impress.js/search) to make sure someone haven't already reported my issue or feature request.

.github/PULL_REQUEST_TEMPLATE.md

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+**Make sure these boxes are checked before submitting your Pull Request, thank you!**
+
+- [ ] I have read and complied with the [Pull Request Contributing Guidelines](https://github.com/impress/impress.js/blob/master/CONTRIBUTING.md#pull-requests).
+- [ ] I confirm there is [an issue](https://github.com/impress/impress.js/issues) for my Pull Request.
+- [ ] I have created a [topic branch](https://github.com/dchelimsky/rspec/wiki/Topic-Branches).
+- [ ] I have used the [Search Tool](https://github.com/impress/impress.js/search?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen&type=Issues) to make sure there are no open Pull Requests fixing the same thing.