Added OAuth authentication, Project.searchAssignableUsers

[beders]

This is a major change and it's fine if you reject it.

It adds several dependencies to get JiraClient to support OAuth via fernandezpablo85/scribe-java (which required RestClient to become an abstract base class).

[bobcarroll]

Thanks for implementing OAuth support! Despite the large change, I think there's a lot of value in including it. Can you rebase against master? Also please add this to the pom:

<properties>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
</properties>

Also, I'd like @chaplinkyle to test attachments since he wrote that code.

[beders]

I'll have more changes in my local version I need to merge as well.