Implemented gson and messages for drop.

* New class: DropSerializer
* New class: DropDeserializer
* New class: DropTypeAdapter
* New class: DropMessage
* New class: DropSerializerTest

The test is skipped because there is no *real* message type right now.

Needed for #24 again.
Reworked key structure with primary and encryption/signature sub-keys.
Added RSA encryption/decryption method.
Merge pull request #43 from jan-schreib/drop-gson

Implemented gson and messages for drop.
Fixed QblEncPublicKey variable name
Merge pull request #45 from L-Henke/keyPairs

Reworked key structure with primary and encryption/signature sub-keys.
#27 Introduced Contact/Contacts class.
Adapted new key api
added primary public key to Contact
Merge pull request #38 from aBothe/pr2

Introduced Contact/Contacts class.
Let an Identity object store both its signature/encryption public/private keys with QblKeyPair-Objects, not primitive strings.
Adapted Identity to new key API
Let Identity's 'id' field be mandatory to be set only once via ctor.
no need for sign/encKeyPairs in Identity
Merge pull request #37 from aBothe/pr

Identities should provide both public&private sign/encryption keys
Made DropSerializer test working.

Forgot how to use embedded classes...
remove unneeded imports
Added LICENSE file
changed sourceCompatibility to 1.7
Update class diagram
Added en- and decryption