The certifier package is renamed to light. This is more descriptive
especially in the wider blockchain context. Moreover we are building
light-clients using the light package.

This also adds godocs to all exported functions.

Furthermore it introduces some extra error handling. I've added one TODO
where I would like someone else's opinion on how to handle the error.

1. +0

   -0
     light/client/main_test.go
2. +21

   -18
     light/client/provider.go
3. +4

   -4
     light/client/provider_test.go
4. +6

   -3
     light/commit.go
5. +2

   -2
     light/doc.go
6. +11

   -5
     light/dynamic.go
7. +9

   -9
     light/dynamic_test.go
8. +8

   -2
     light/errors/errors.go
9. +0

   -0
     light/errors/errors_test.go
10. +12

    -10
      light/files/commit.go
11. +2

    -2
      light/files/commit_test.go
12. +14

    -9
      light/files/provider.go
13. +10

    -10
      light/files/provider_test.go
14. +13

    -11
      light/helpers.go
15. +23

    -10
      light/inquirer.go
16. +21

    -23
      light/inquirer_test.go
17. +10

    -5
      light/memprovider.go
18. +17

    -17
      light/performance_test.go
19. +21

    -43
      light/provider.go
20. +39

    -18
      light/provider_test.go
21. +8

    -3
      light/static.go
22. +7

    -7
      light/static_test.go