Implement incremental prepare

[tzraikov]

Currently all prepare steps are executed on every run and debug command. This process can be improved:

When executing tns run, tns debug or tns livesync:

• Changes in app folder (ts,js,xml,css files) should not trigger prepare task
• Changes in app resources should trigger prepare task
• Changes in modules should trigger prepare task
• Build task will be triggered only when a prepare task was executed (or when the target device has changed for iOS)

Other commands:

• tns prepare should always execute the full prepare task
• tns build should always execute the full prepare and build tasks