Hockey CrashManager lib

This change allows the Hockey Crash Manager to be used on its own. In our environment, we didn't need the update manager or hockey manager and wanted to simplify our integration, so we created a new HockeySDK target (HockeyCrashManagerLib) to build a HockeyCrashManager static library containing only BITCrashManager. The code is refactored so that BITCrashManager can be used on its own, with no dependencies on BITHockeyManager or BITUpdateManager.

Why Hockey CrashManager lib (in more detail) ?

 * We didn't want HockeyManager:
   * it is a singleton, something we try to avoid
   * it creates both crash manager & update manager objects, whether you want to use them or not
   * it included code to detect and call Jira Mobile Connect, part of which caused us an iTunes Connect validation warning (@selector(ping))
   * we wanted to simplify our integration by only using BITCrashManager directly, we didn't need the update manager

 * libHockeyCrashManager.a embeds PLCrashReporter so our app doesn't need to know about it.

 * Debug log switching for BITCrashManager and BITUpdateManager is controllable by their own debugLogEnabled property, so they can be used without BITHockeyManager. BITHockeyManager will propagate any debugLogEnabled flag changes down to these objects.

Let me know what you think.
Cheers,
Chris