Use constants for tls protocol values.

Instead of hardcoding the values use constants like Alert.LEVEL.FATAL or
Extension.TYPES.SERVER_NAME.

This doesn't create constants for all TLS constants, there are still
more to be done.
Merge pull request #49 from google/enum

Use constants for tls protocol values.
More TLS alert description values
Add TLS record and handshake protocol enums
Rename Extension.TYPES to TYPE for consistency/style
Merge pull request #52 from google/types-type

Rename Extension.TYPES to TYPE for consistency/style
Merge pull request #50 from google/more-alert-values

More TLS alert description values
Merge pull request #51 from google/record-and-handshake-enums

Add TLS record and handshake protocol enums
Whitelist invalid_protocol alert in early CCS test