Use Connection String from `<connectionStrings>` in addition to `<appSettings>`

In .NET 3.5 and newer, the preferred way to store connection strings is:

```xml
<connectionStrings>
    <add name="myConnectionString" connectionString="..." />
</connectionStrings>
```
and to read using:

```c#
ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString
```

instead of the old way:

```xml
<appSettings>
    <add key="myConnectionString" value="..." />
</appSettings>
```
and to read using:

```c#
ConfigurationSettings.AppSettings.Get("myConnectionString")
```

Because many other components (for example, [MongoRepository](https://mongorepository.codeplex.com/)) that use the new method of specifiying connection strings can be used alongside this project, it would be nice if we can specify the connection string in only one place for all components that we use.

In order to maintain compatibility with old config files, this pull request checks *both* locations for the connection string (preferring the newer one).

In the process, I also changed it so all three classes that need the connection string use the same code (moved to the Util class) to find it.