Fixed exception for addons with less than 2 tags

This commit fixes #4, two excpetions regarding UpdateMetadata: one for addons
with not exactly 2 tags and the other for null-reference when saving an addon
with empty value.
Adding "ServerContent" tag
Removed printing error message directly from reader
Implemented multiple file export and remove capability
Fixed autoselecting addon file's directory for LegacyExtract
Added license