Add a more descriptive rest-client user agent.

Identify ourselves as rest-client and also include system architecture
and ruby / ruby engine version information.

For example:

    User-Agent: rest-client/1.7.2 (linux-gnu x86_64) ruby/1.9.3p550

    User-Agent: rest-client/1.7.2 (linux x86_64) jruby/1.7.9 (1.9.3p392)
Merge pull request #324 from rest-client/user-agent

Add a more descriptive rest-client user agent.