[JENKINS-22834] Support additional classpaths

[JENKINS-22834] (https://issues.jenkins-ci.org/browse/JENKINS-22834)

[Groovy postbuild plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Groovy+Postbuild+Plugin) provides a feature to define additional classpath.
This request adds an alternate feature to script-security plugin.

* You can define additional classpaths for groovy scripts.
    ![script-security-classpath1](https://cloud.githubusercontent.com/assets/3115961/3789755/a110cbcc-1abf-11e4-9608-dfc29e12a3ea.png)
* Classpaths fields are hidden (advanced section) by default
    ![script-security-classpath2](https://cloud.githubusercontent.com/assets/3115961/3789759/a77bfbb2-1abf-11e4-8ad5-0eea92eb3db6.png)
* You can approve classpaths in In-process Script Approval page.
    ![script-security-classpath3](https://cloud.githubusercontent.com/assets/3115961/3789758/a567cac2-1abf-11e4-9eea-360d3c070a9b.png)

Classpaths are identified with their SHA1 hashes.
If a classpath is a directory (that is, classes directory), a SHA1 hash is calculated using all files contained in that directory.