Vehicle connected sensor, bug fix & deprecation resolution

[zackslash]

Hi!

This pull request adds a new sensor, fixes a pre-existing bug and resolves some deprecation warnings.

• Adds a new sensor to indicate if a vehicle is connected to the charger (Using the same logic as the app, as a session is started when a vehicle is connected)
• Fixes a bug where some entities would become 'unavailable' and the integration needed to be reloaded (Resolves Lock state unknown #16), this was due to the charging_time sensor being set to a state_class of 'total_increasing' while also defining 'last_reset', (this is unsupported in HA).
• Resolves a number of deprecation warnings for recent versions of home assistant

I've been running this branch for a while, happy to answer any questions.

[zackslash]

Need to fix the tests, will re PR this once ready