Fix crashes when source files are empty.

Check for nil more often.
Updated specs and fixtures.
Empty files also appear in cobertura xml report.
Fixes and corrections of last pull request.

Correct and simplify code in rate_lines_tested and percentage_lines_tested.
Method gcov_data returns an empty string instead of nil.
Removed checks in nil where the empty string gets processed.
Returning empty array for empty branch_coverage_data_for_statement_on_line values.
Updated spec.
Merge branch 'master' of https://github.com/venmo/slather
Update CHANGELOG.md
Merge pull request #34 from tarbrain/master

Fix crashes when source files are empty.