Duplicate EveCharacter models being created

MultipleObjectsReturned: get() returned more than one EveCharacter -- it returned 2!

Best guess is this was introduced with #591 - somewhere they're being created without checking if they exist correctly. Type mismatch?

While we're at it, the following fields should become unique:

• EveCharacter.character_id
• EveCharacter.character_name
• EveCorporationInfo.coporation_id
• EveCorporationInfo.corporation_name
• EveAllianceInfo.alliance_id
• EveAllianceInfo.alliance_name
• EveApiKeyPair.api_id