implement transaction and client services
add payment service
restful refactoring
fix typo
check if fee is null before get its attributes
add offer and refund services
complete client functonality
client and offer model + service + javadoc
Payment model + service
code enhancements
add WebhookService,
add all tests,
return PaymillList,
add multithreading support
reorganize classes
mate tests state independent
tuning, add License and README
pass private key as sys property
externalize API KEY
v3
update pom dependencies
refresh instance on show and update
update and delete return void
add documentation
Merge remote-tracking branch 'origin/master' into version3.0

Conflicts:
	.gitignore
	pom.xml
	src/main/java/com/paymill/context/PaymillContext.java
	src/main/java/com/paymill/models/Client.java
	src/main/java/com/paymill/models/Fee.java
	src/main/java/com/paymill/models/Offer.java
	src/main/java/com/paymill/models/Payment.java
	src/main/java/com/paymill/models/Preauthorization.java
	src/main/java/com/paymill/models/Refund.java
	src/main/java/com/paymill/models/Subscription.java
	src/main/java/com/paymill/models/Transaction.java
	src/main/java/com/paymill/models/Webhook.java
	src/main/java/com/paymill/services/ClientService.java
	src/main/java/com/paymill/services/OfferService.java
	src/main/java/com/paymill/services/PaymentService.java
	src/main/java/com/paymill/services/PreauthorizationService.java
	src/main/java/com/paymill/services/RefundService.java
	src/main/java/com/paymill/services/RestfulUtils.java
	src/main/java/com/paymill/services/SubscriptionService.java
	src/main/java/com/paymill/services/TransactionService.java
	src/main/java/com/paymill/services/WebhookService.java
	src/test/java/com/paymill/services/ClientServiceTest.java
	src/test/java/com/paymill/services/OfferServiceTest.java
	src/test/java/com/paymill/services/RefundServiceTest.java
	src/test/java/com/paymill/services/SubscriptionServiceTest.java
	src/test/java/com/paymill/services/TransactionServiceTest.java
	src/test/java/com/paymill/services/WebhookServiceTest.java
improve README and models doku