Upgrade to io.dropwizard 0.7.0; add support to load servlets in dropwizard environment

This pull request includes merges from two other pull requests (still open):
- https://github.com/hmsonline/dropwizard-spring/pull/11 - load servlet configs into dropwizard environment;
- https://github.com/hmsonline/dropwizard-spring/pull/10 - upgrade to io.dropwizard 0.7.0;

Additional changes:
- upgrade dependencies and plugin to latest compatible release
- update README