Use fallback `Context` if one was not specified. (Fixes #186)

[XerTheSquirrel]

If a Context was not passed to the method it will use one which is derived from environment variables.

[XerTheSquirrel]

Environment variables of interest, that we can use to recover information that should be in the context:

• AWS_ACCESS_KEY_ID
• AWS_DEFAULT_REGION
• AWS_EXECUTION_ENV
• AWS_LAMBDA_FUNCTION_MEMORY_SIZE
• AWS_LAMBDA_FUNCTION_NAME
• AWS_LAMBDA_FUNCTION_VERSION
• AWS_LAMBDA_LOG_GROUP_NAME
• AWS_LAMBDA_LOG_STREAM_NAME
• AWS_REGION
• AWS_SECRET_ACCESS_KEY
• AWS_SESSION_TOKEN
• AWS_XRAY_CONTEXT_MISSING
• _AWS_XRAY_DAEMON_ADDRESS
• AWS_XRAY_DAEMON_ADDRESS
• _AWS_XRAY_DAEMON_PORT
• _HANDLER
• LAMBDA_RUNTIME_DIR
• LAMBDA_TASK_ROOT
• _X_AMZN_TRACE_ID

[XerTheSquirrel]

Okay, not sure about these:

• String getAwsRequestId()
• String getInvokedFunctionArn()
• int getRemainingTimeInMillis()

From my list of variables, these are not exposed.

[XerTheSquirrel]

For String getAwsRequestId() I just derived this from the hashCode(), it is only 32-bits but it is better than a constant.