Fix crash when block fails without error (fix pixelglow#129)

* The NSUnderlyingErrorKey value would be nil in the userInfo dictionary initialization, causing the crash. To fix, generate a ZZBlockFailedWithoutError error to substitute for the missing underlying error.

* Update unit tests.

* Update podspec to 8.1.1.

Add fine-grained filename encoding

* Add entry encoding property based on the EFS flag of the general purpose bit flags.

* Add entry fileNameWithEncoding: methods to force a particular entry-level encoding for the file name.

* Remove archive-level encoding for file name.

* Default encoding is now CP-437, as per the Zip file format specification. This is a breaking change since previous default encoding was UTF-8.

* Update unit tests.

* Update podspec to 8.1.

Fix missing nullables + generics from @8a28415

* Update podspec to 8.0.6.

Add nullability annotations and lightweight generics

* Require Xcode 7 to build.

* Update podspec to 8.0.5 (fix pixelglow#117).

Fix podspec capitalization

* Can't change the capitalization of pod specs. We're stuck with "zipzap".

Fix pod link fail due to missing C++ symbols (fix pixelglow#92)

* Fix the unexpected libc++ dependency.

* Prevent C++ build warning with a separate C++ subspec.

* Update pod spec to 8.0.3.

Fix pod lib lint errors (fix pixelglow#78, pixelglow#86)

* Fix Objective-C++ build errors: headers not found.

* Fix Objective-C++ build warning: unused argument.

* Update podspec to 8.0.2.

Merge pull request pixelglow#73 from laiso/public_headers

Fix pixelglow#71 : Add headers to podspec.

Improve archive init API

* No more separate mutable archive subclass (ref pixelglow#24).

* Initializers and factory methods may now return nil, along with error information.

* Initializers now take an options dictionary, which allows for expansion. For now, you can set ZZOpenOptionsEncodingKey for string encoding and ZZOpenOptionsCreateIfMissingKey to create the archive file if it is missing.

* Drop the public load: method, as initializers and updateEntries: do any necessary loading. It’s fairly cheap to create a new archive object to reload changed contents, and we no longer lazily load contents and entries.

* The redesign prevents creating erroneous objects e.g. file not found may be a legitimate problem, and prevents swallowing of read errors during lazy loading.

* Update podspec to version 8.0.

Add ZZConstants.h to public headers