Use require-dir to pull in tasks automatically

Addresses #7 

This PR aligns the project with the "blessed" way to require tasks as per the Gulp project. Two things had to happen for this to work:
1. Use `require-dir` to auto require any task placed under tasks.
2. Removes the export syntax from the task files. 

NOTE: This PR is best viewed without whitespace changes. Use the query param `?w=1` to do that.
`requireDir` is undefined.