Changed `id` return type to `instancetype`
Prevent a potential crash.

- It appears that allocating and autoreleasing a CTTelephonyNetworkInfo from a class method causes a dereferencing exception, which causes a crash. This only happens on devices with an active and valid data service (only tested on iPhones but it might also affect iPads with cell data network access) by turning off Airplane mode and turning it back on and switching quickly to the app.
Added methods to remove super properties that have been previously registered, for use as an alternative to clearing all of the super properties just to change a subset of them.
Ensure that a background task is started whenever -flush is called while the app is in the background. This ensures that the app can flush events within its own background tasks while maintaining the behavior of -flushOnBackground automatic flushes.
setOnce
IDFA property
use IDFA as default distinct id
use ODIN1 as fallback default distinct id
unified identify:
add ODIN as a lib
add $app_short_version_string
Merge pull request #57 from Newstex/pull-requests/delayed-background-upload-fix

Regression: -flush will not complete upload if called while app is in the background
Merge pull request #54 from RebelVox/master

Rare Crash in the `deviceInfoProperties` Method
Merge pull request #56 from Newstex/pull-requests/unregister-super-properties

Added ability to remove previously registered super properties
Merge pull request #53 from markonikolovski/instancetype

Changed `id` return type to `instancetype`
Merge pull request #48 from andrewschaaf/fix-whitespace-typo

Fix a whitespace typo
Merge branch 'master' of github.com:mixpanel/mixpanel-iphone

Conflicts:
	Mixpanel/Mixpanel.m
make unregisterSuperProperty: the same as other libs and tests
fix up tests
bump version number