Make getMultipleRecipientServiceAddress more robust

to avoid

feature-not-implemented
	at org.jivesoftware.smack.PacketCollector.nextResultOrThrow(PacketCollector.java:182)
	at org.jivesoftware.smack.PacketCollector.nextResultOrThrow(PacketCollector.java:161)
	at org.jivesoftware.smackx.disco.ServiceDiscoveryManager.discoverItems(ServiceDiscoveryManager.java:573)
	at org.jivesoftware.smackx.address.MultipleRecipientManager.getMultipleRecipienServiceAddress(MultipleRecipientManager.java:310)
 	at org.jivesoftware.smackx.address.MultipleRecipientManager.send$7a184d34(MultipleRecipientManager.java:82)

for e.g. Google Apps XMPP services.

This is not to be merged in master/4.1 branch as it already contains a
similar fix.
Fix extensions/intro.html filetransfer link
Add Enable/Disable Carbon IQs
Add StanzaIdTest
Make Bookmarks.toXML() return valid XML again

Fixes SMACK-594
Generify Packet class returned by PackageCollector everywhere

This will help to get rid of repetitive class casts, and make
PacketCollector api more inline with itself (since some methods
are already generic return methods).
Enable more 'checkstyle' checks

AvoidStarImport
IllegalImport
RedundantImport
UpperEll
ArrayTypeStyle
GenericWhitespace
EmptyStatement
PackageDelcaration
Fix faulty string comparison in SubscriptionProvider

'!=' tests for reference inequality, must use '!String.equals(String)'
instead.
Add an API to send and wait async for a response

SMACK-595

Also remove the unused AbstractXMPPConnection.getPacketListeners()
method.
Convert html documentation to markdown
Smack 4.0.3
Merge branch '4.0'

Conflicts:
	build.gradle
	documentation/connections.html
	documentation/debugging.html
	documentation/extensions/filetransfer.html
	documentation/extensions/rosterexchange.html
	documentation/gettingstarted.html
	documentation/index.html
	documentation/legacy/messageevents.html
	smack-extensions/src/main/java/org/jivesoftware/smackx/address/MultipleRecipientManager.java
Merge branch 'master' into xep198

Conflicts:
	smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java