Add sendThreads property to BugsnagConfiguration

[fractalwrench]

Goal

Adds the sendThreads to BugsnagConfiguration. This allows a user to record thread information always, for unhandled errors only, or never.

Note that the stacktrace of the crashing thread is always recorded and reported in the thread tab.

Changeset

• Added publicly visible BSGThreadSendPolicy enum and sendThreads property
• Passed value of sendThreads into KSCrash during its initialisation
• Altered KSCrash to take an int rather than a bool for the threadTracingEnabled value
• Added logic to only serialize all threads if the user has specified to do so

Tests

• Added unit tests to verify default value and copying of thread information in BugsnagConfiguration
• Manually verified behaviour in an example app

[robinmacharg]

Looks good. Couple of minor nits, approved on the basis that you'll use best judgement on these.