Add new functions to work with Eurostat codes and labels/definitions

Add new functions to work with Eurostat codes and labels/definitions
The function label_eurostat takes Eurostat codes and turn them to 
labels with getEurostatDictionary. label_eurostat_vars and 
label_eurostat_tables are special cases for variable names and 
table names.

Handy for example when plotting.

Issues:
- Now uses mapvalue from plyr. Could be implemented also without.

- Naming. I tried to follow Hadley’s style guide as advised in rOpenGov 
  guidelines. Other functions in package are however in camelCase.
  Should I change?

- Documenting is still minimal.