Fix import for Dropdown fieldtype

Importing data into a dropdown field in Craft 2.2.2591 on PHP 5.6 triggers an array to string exception when saving the updated entry.
 
Updated the prepForFieldType function to keep dropdown data as a sting, as well as including code which maps the correct field value if the CSV contains dropdown field labels.
Merge pull request #16 from code418/patch-1

Fix import for Dropdown fieldtype
Update Plugin and README