add "assert_string_equals" that diffs multiline strings

Test cases that need to compare large multiline strings
should probably use assert_string_equals, as it will
show a unified diff between the two strings it is
checking for equality.

Note that python2.7+ does this automatically for
assertEqual for strings.
use "assert_string_equal" for multiline str asserts

Also, some minor import order refactoring.
Merge pull request #568 from candlepin/alikins/assert_string_equals

Alikins/assert string equals