Updated Acceptance Tests

- Logging in with LMS before all tests
- Skipping error page tests if the pages are not enabled
Display empty previous data point on trend line when only one data point exists.
Merge pull request #157 from edx/dsjen/update-sparse-trend-display

Display empty previous data point on trend line when only one data point exists.
Merge pull request #155 from edx/lms-login-acceptance-test

Updated Acceptance Tests
Updated Preface for new docs
Merge pull request #159 from edx/markhoeber-preface-update

Updated Preface for new docs
Run Acceptance Tests on Travis
TE-497