Add support for contextual _meta on all BTClient posts

* Add BTClientMetadata and BTClientMutableMetadata
* Implement NSCopying for BTClient, BTHTTP, BTClientToken
* Introduce BTClient `copyWithMetadata` to create client instances with
altered metadata
* Customize metadata in drop-in.

https://trello.com/c/XdTkIFbl
Add copyWithMetadata: specs, move all to BTClient+MetadataSpec.h

https://trello.com/c/XdTkIFbl
Clean up tests related to BTClient with metadata parameters
Replace BTClient Metadata category with a class extension
Add pending specs around isEqual: for classes that newly implement copying
Rename basePostParameters -> metaPostParameters method
Remove cruft in BTClientMetadata header
Merge remote-tracking branch 'origin/master' into feature/add-meta-param-to-btclient-posts

Conflicts:
	Braintree/API/Braintree-API-Specs/BTClientSpec.m
Fix remaining BTClient_Metadata imports
Add `integration` and `source` analytics params

https://trello.com/c/XdTkIFbl
Rename BTClient+MetadataSpec -> BTClient_MetadataSpec
Merge branch 'feature/add-meta-param-to-btclient-posts'
Remove unneeded import
Accommodate iOS 8 deprecations

* `kCLAuthorizationStatusAuthorized` -> `kCLAuthorizationStatusAuthorizedAlways`
* `UIViewController` interface orientation methods
* `NSGregorianCalendar` -> `NSCalendarIdentifierGregorian`

https://trello.com/c/8HWCffkd
Fix typos
Remove duplication