Rename to get_eurostat and tidy_eurostat

The old get_eurostat to tidy_eurostat.
The read_eurostat to get_eurostat
Changed tidy_eurostat and get_raw_eurostat to internal functions.
Modifed documentation to reflect name changes.
Removed default from id in tidy_eurostat.
get_eurostat to call get_eurostat_raw and tidy_eurostat

tidy_eurostat no longer calls get_eurostat_raw put takes a data.frame returned by get_eurostat_raw as an input. get_eurostat now calls first the get_eurostat_raw and then the tidy_eurostat.

Small changes to documentation.
Merge pull request #12 from jhuovari/master

get_eurostat to use cache