Try improve code for configuration value references

[vajda]

Currently, configuration support value references, for example:

user:
  firstName: Nick
  lastName: Smith
someField: $user.firstName

This feature is useful, but code implementing it can be improved. Both ValueExpressionParser and DataGenerator classes have same private method stripOffLastReference and logic for constructing full name and usage of '.' character is spread through these two classes. This should probably be moved into separate class which both classes would use.