[feature][mbed] Set `XI_USER_AGENT` -

Simply append it to `xi_consts.h`, as there is no global constant which we can rely on.
The issue had been raised with mbed team and hopefully `__MBED__` will soon appear!
[build][fix] Revision is already known
Merge pull request #11 from xively/mbed_user_agent_string

[feature][mbed] Set `XI_USER_AGENT` -