Modified SqlServerStorage constructor to take name or connection string parameter

For #51 I modified the constructors on `SqlServerStorage` to take a single string parameter that is either a full connection string or the name of a connection string from the config file.  The constructor figures out which is provided based on presence of a semicolon within the argument.  I had to add a dependency on `System.Configuration` to `HangFire.SqlServer` in order to have access to `ConfigurationManager`.  

I didn't see any tests around `SqlServerStorage`, I can add some if so desired.  Also wasn't sure of the commenting policy, but I did add some comments to the primary constructor.  Let me know what changes you'd like to see and I can make them.