Created `.gitignore` file.

Closes #25
* Ignores the node_modules directory because it's not needed in the repository.
* Ignores the DS_STORE file on Mac because it's pointless committing that every time.
* Ignores npm log file because it's unnecessary to store that in the repository.
Merge pull request #32 from ryansmith94/patch-1

Created `.gitignore` file.