Extended error handling and added support for finding admin by ID

[tanasegabriel]

Added support for the following error codes:

• client_error
• type_mismatch
• not_restorable
• server_error
• token_not_found
• token_revoked
• token_blocked
• token_expired
• admin_not_found

Reference - https://developers.intercom.com/v2.0/reference#error-codes
Fix for #161

Added support for finding admins by id (fix for #159):
intercom.admins.find(id="123456")

[coveralls]

Coverage increased (+0.01%) to 95.842% when pulling fcdce48 on tanasegabriel:master into abcc272 on jkeyes:master.

[coveralls]

Coverage increased (+0.01%) to 95.842% when pulling fcdce48 on tanasegabriel:master into abcc272 on jkeyes:master.

[jkeyes]

@tanasegabriel Can you revert your changes to travis.yml and to admin.py as I've merged those from other PRs. I'll then be able to merge the additions you've made to the error handling module.

[coveralls]

Coverage increased (+0.02%) to 95.854% when pulling 8151508 on tanasegabriel:master into abcc272 on jkeyes:master.

[tanasegabriel]

@jkeyes Sure thing!

[coveralls]

Coverage increased (+0.02%) to 95.854% when pulling ec699cf on tanasegabriel:master into abcc272 on jkeyes:master.