Add a custom prefix to KSCrash to avoid conflict, improve pod specification

[kattrali]

Changes

• Updated KSCrash to latest upstream changes, applying Bugsnag's patches on top
• Adds a Bugsnag prefix to vendored classes from KSCrash
• Updated the podspec to use the new dependency as well as moving the target configuration to cocoapods-bugsnag, a cocoapods plugin.

Rationale

Until we/someone has the bandwidth to develop a solution to kstenerud/KSCrash#84, this change allows client applications to simultaneously use other libraries dependent on KSCrash. The namespacing is done via a script committed to a patch of KSCrash, and it finds and updates unprefixed classes as needed.

Fixes #41, #52, #72

[snmaynard]

LGTM