make cache import include .gitignored files

Summary:
`git add -A` doesn't add ignored files by default. But *peru* doesn't have any
concept of ignored files. When peru imports a directory, it should import
eveything.

https://github.com/oconnor663/peru/issues/15

Test Plan: New unit test.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D12
create shared.write_files()

Summary: This saves us from manually opening lots of files in tests.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D15
coveralls support

Summary: Integrate with coveralls.io.

Reviewers: jacko

Reviewed By: jacko

Differential Revision: https://phabricator.buildinspace.com/D28