[stackexchange] Support the use of access_token

[Jose Javier Merchante]

Currently StackExchange supports two options to throttle requests:

• Application key
• Application key + access_token

The first option is supported by Perceval, but we are using Grimoirelab with multiple users and it would be nice to support access_token to have a different quota per user.

We will provide a Pull Request with the changes required.

Documentation:

• https://api.stackexchange.com/docs/authentication ('Usage' section)
• https://api.stackexchange.com/docs/throttle