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
Make Bookmarks.toXML() return valid XML again

Fixes SMACK-594
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