User specified memory usage and timeout warning thresholds

[aspotton]

Added two config options to allow for configuring the thresholds to trigger the timeout and memory usage events. The default values fallback to the current hard-coded ones if these options aren't specified.

config options:
timeout_warning_threshold (environment variable: SENTRY_TIMEOUT_THRESHOLD)
memory_warning_threshold (environment variable: SENTRY_MEMORY_THRESHOLD)

These are float values that represent the percentage for the threshold trigger.

For example: To have a timeout warning at 30 seconds on a function with a max runtime of 60 seconds, set timeout_warning_threshold = 0.50

[coveralls]

Coverage decreased (-0.2%) to 89.057% when pulling 0cb202c on aspotton:set_warning_thresholds into f06e664 on Netflix-Skunkworks:master.