Proxy service with Spring Framework. Partial #13 issue

Common interface for http-proxy service and implementation (fixes #7 issue):

* Spring dependencies has been added to pom.xml
* Load Spring context on servlet init.
* Proxy configurator as service

Proxy configuration improvements:

* Proxy configurator implementation that loads proxy properties externalized. Fixes #8 and #6

More proxy library improvements:

* Proxy configurator implementation that loads proxy properties at runtime with common-configuration library. It load configuration from multiple files ('classpath:proxy-*.properties' pattern).
* Library classes refactored adding some packages
* Proxy callbacks as services configured in Spring context.
* Junit test mocked to test http-proxy service alternatives.
* Changes in old junit test.