Refactored MavenCentral interface

Renamed the MavenCentral interface to DependencyResolver (which is a more
accurate description of its purpose) and renamed its implementing classes to
MavenResolver and CachedMavenResolver.
Added ability to run Robolectric tests offline

Set the following system properties to activate the behavior:
  robolectric.offline=true
  robolectric.dependency.dir=<path>

Runtime dependencies will be retrieved from the specified path
instead of being downloaded from Maven.
Further renaming for consistency
Removed RobolectricTestRunner from DependencyResolver methods

Moved the configureMaven() method from RobolectricTestRunner to
MavenDependencyResolver, since it is only applicable there. This
means that DependencyResolver methods no longer require an
instance of RobolectricTestRunner.

THIS IS AN API-BREAKING CHANGE.

Instead of sub-classing RobolectricTestRunner to provide custom
Maven configuration, it is now necessary to subclass
MavenDependencyResolver.
Merge pull request #1189 from awrichar/offline

Added ability to run Robolectric tests offline.