Add a filesystem interface, mock, and tests for the mock.

This allows testing sysfs based drivers by swapping out the real
filesystem for a mock one.

Signed-off-by: Michael Hope <mlhx@google.com>
Refactor sysfs to use internal fs
Add syscall mock
Refactor raspi to use new sysfs interfaces
Refactor edsion tests
Improve error message
Refactor beaglebone tests
Refactor beaglebone tests
Update .travis.yml
Update travis build script
Merge pull request #144 from hybridgroup/fs_mock

sysfs fs interface