Stripped out MultiScreen discovery for next 1.3.x release

MultiScreen will still be present in the sdk_1.4 branch, but since we are backporting a ton of changes to a 1.3.2 release, we are pulling it for now.
Better handling of media state w/ multiple clients
Fixes #88, issue w/ web app launch in CastService
Changed version for next release
Updated changelog
Updated podspec for next release
Merge branch 'sdk_1.3'
Swapped source files for submodules
Added compiler flags, reference to prefix file
Changed framework path to modules dir, recursive
Moved ConnectSDK.h into core
Added default platforms constant
Fixed bad reference to ConnectSDK.h
Made ConnectSDK.h a public header
Update submodules for recent project file changes
Fixed typo in default platforms list
Updated readmes to include directions for modules
Merge remote-tracking branch 'origin/modularization' into sdk_1.4-dev
Merge remote-tracking branch 'origin/master' into sdk_1.4-dev

Conflicts:
	ConnectSDK.xcodeproj/project.pbxproj
	ConnectSDK/ConnectSDK-Prefix.pch
	ConnectSDK/Discovery/DiscoveryManager.m
	ConnectSDK/Services/CastService.m
	ConnectSDK/Services/Sessions/CastWebAppSession.h
	ConnectSDK/Services/Sessions/CastWebAppSession.m