use stringr library to proccess strings
Improved generality

- check that the directory exist in a system-agnostic way
- better `getCSV` function using httr
Merge branch 'master' of https://github.com/tpoisot/DB_EU_SubImport into tpoisot-master

Conflicts:
	script_get_american_data.R
	script_merge_american_data.R

	modified:   script_merge_american_data.R