Added public Contact ID field and findOne() method

[alexbatashev]

• Added public Contact ID field
• Added findOne() method

Contact ID allows to query specific contacts.
findOne() method returns the first contact in the list or null (useful with contact id).

[tamir7]

1. ContactID already exists in InternalFields. If you need it in regular fields, you should remove it from Internal.
2. Why do you use org.jetbrains.annotations.Nullable and not android.support.annotation.Nullable?
3. I like that you fixed the gradle build (BINTRAY_USER and BINTRAY_API_KEY) but you broke my BintrayUpload..