Adding the modified public constructor to DiscoveryClient

Fix for issue #174 added another argument to the constructor used by guice.
This was considering the fact that code should not be using the @Inject constructor directly. However, some code inside Netflix was dependent on it.

 Adding the old constructor back and making the @Inject constructor package private.
Merge pull request #179 from NiteshKant/master

Adding back the modified public constructor to DiscoveryClient