Document executeWithTiming and change behavior to assert that metricName and block are not nil.
Document recordStart method, assert that metricName is not nil.
Document recordFinish and assert that metricName is provided.
Document recordFinishAndSendMetric and assert that metricName is not nil.
Document sendMetric and assert metricName and metricValue exist.
Document proper use of initSharedLogger and initWithConfig.  Assert apiKey provided to initiSharedLogger is not nil.
Merge branch 'issue-6-api-docs-and-assertions'

Closes issue #6