Make repositoryUrl optional for dependency:get

[Note : could "get" component be created in this JIRA? Thanks]

I think dependency:get could be made simpler to use. -DrepositoryUrl is a required parameter although it's a information that can be retrieved automatically from the current settings.xml environment.

repositoryUrl parameter could be made optional so that user can redefine the current effective-settings environment, but required makes it less fluent to use.

Cheers