Add feature: more entries of the call stack could be printed out on assert failure

Hi,

I add some codes for a configurable feature: more entries of the call stack could be printed out on assert failure.
 Refer to [this issue](https://github.com/stretchr/testify/issues/81#issuecomment-54835227) for more info.

The previous two commits in this PR are unrelated since they are just reformats(my code editor would replace all tabs to spaces on saving the file). Please refer to the latest commit for the modification.