added setting to enable specifying the user-agent header
Set default user-agent

Use the gem name and version as the user agent.
Fix method call

http_extra_header is not a valid message of xmlrpc client
Merge pull request #31 from nateleavitt/bb-user-agent

User-Agent
1.1.8
Merge branch 'master' of github.com:nateleavitt/infusionsoft