Add config settings for admin and application context path

This changeset adds configuration settings for the admin context path and
the application context path to DefaultServerFactory. It simplifies changing
the respective context path under which the respective handler can be accessed.

Fixes #602.
Add config setting `rootPath` for the Jersey URL pattern

The configuration setting `rootPath` for defining the Jersey URL pattern
existed in Dropwizard 0.6.x but was removed. Since there has been some
confusion around `applicationContextPath` (which does something different)
and the old `rootPath` setting, it seems to be best to re-add the setting
to AbstractServerFactory.
Clarify documentation for `rootPath`
Merge pull request #665 from joschi/issue-602-context-paths-for-defaultserverfactory

Add application and admin context paths to DefaultServerFactory, fixes #602