Create structured BugsnagStackframe class

[fractalwrench]

Goal

Creates a publicly accessible BugsnagStackframe class which holds fields in properties, rather than an NSDictionary.

A project-visible BugsnagStacktrace has been created to sanitise the stacktrace - notably by trimming its length to <200 frames, as per the spec.

The BugsnagStackframe class will eventually be present on the BugsnagError and BugsnagThread classes, which will be implemented in future PRs. For now the JSON serializer has been updated to use the object internally.

Changeset

• Add properties to BugsnagStackframe which can be edited by the user. Note that these deviate from the spec
• Create BugsnagStacktrace which limits the number of BugsnagStackframe to <200, and encapsulates stacktraces to allow any similar modifications in future
• Updated BugsnagEvent JSON serialization to use BugsnagStacktrace internally
• Refactored BSGFormatFrame to serializers in BugsnagStackframe/BugsnagStacktrace

Tests

Added unit tests to verify that stacktraces can be parsed from a dictionary/serialized to JSON. Relied on existing unit test coverage to verify the event JSON remains unchanged.

[robinmacharg]

Looks good. Couple of comments very similar to the event.device ones, unsurprisingly given the sequence of commits. Also, small Xcode nitpick: it looks like the Event class has moved out of sequence to the top of the Payload group.

[fractalwrench]

Addressed inline comments and resolved the XCode source code organisation nit. I've also rebased against v6 to fix the merge conflicts.

[robinmacharg]

LGTM.