Examples: improvements

[Yoav Tock]

Here is a collection of improvements and additions to the current examples.

Refactor

1. The config.go and config.yml seem to be the same on all examples. Can we put a single copy in a package, say examples/util

Asynch and checking validity

1. Add example to show how to use asynch commit
2. Add example to show how to check that a transaction is valid
3. Add example to show the difference between Insert & Put and how to implement them

ACLs

1. Show an example of using ACLs
2. on record
3. on database
4. show a tx that turns out non-valid because of ACL violation