Maven support

This diff add maven build system for UrlImageViewHelper library. Issue #28

1) I've decided not to create parent pom for sample and library since it is not recommended by maven central.
2) This diff may cause some problems under eclipse without m2e and m2e-android. To fix that problem I would recommend to remove .settings and .project (and all other eclipse specific stuff) from version controll. Otherwise you may want to create separate branch where maven support will be maintained.
3) Finally, I had to remove references to junit.framework.Assert since junit libraries should only be used in test scope.