Update licence info to LPGLv3
Merge master
Merge pull request #3 from wallyworld/fix-licences

Update licence info to LPGLv3
Make sure the Trace/Annotate/Wrap functions return nil if the error is nil
Merge pull request #4 from howbazaar/handle-nil

Make sure the Trace/Annotate/Wrap functions return nil if the error is nil
Add an example of writing example tests.
external test
Merge pull request #5 from juju/100-add-trace-example

Add an example of writing example tests.
Fixed outdated import
Merge update to use gopkg.in/check.v1