Adding increment method to Intercom::User
Fixing admin avatar path for MessageAuthor

- We changed the response to be hash of square_25, square_50, and square_128
- Specifically return nil if you look for those fields on an non-admin
Use API's increments hash instead of direct update of custom_data
Fail silently and another test

- Fail silently with non-numeric fields (like the REST API)
- Adding test to make sure we can call increment multiple times
Sum the values of the increments if called in succession
Simpler
Merge pull request #40 from intercom/add-increments-fix-message-threads

Add increments fix message threads