Add facts_to_tags to the main class

The user can use this array to put facts that he'd like to convert
into tags.
The result in Datadog will be: fact_name:fact_value if you put the
string fact_name in facts_to_tags.
Add documentation for facts_to_tags
Merge pull request #41 from DataDog/leo/facts_to_tags

Add facts_to_tags to the main class