Feat: remove library dependency

[mrsofiane]

According to this issue #9, i figured out that to use the library you should also import retrofit library, i made some changes to remove this dependency :

• deprecate the create invoice method.
• replace it with a new submit invoice method.
• create custom callback and custom response.
• add more tests.
• update readme.
• add tag v1.1 to publish changes on jitpack