title: DbContext constructor with connString causes problems with EF Migrations and EF Powertools
body: <p>I would like to pass a connection string to the constructor of DbContext.  The way to do this has been answered here: <a href="https://stackoverflow.com/questions/4805094/pass-connection-string-to-code-first-dbcontext">Pass connection string to code-first DbContext</a></p>
  
  <p>(code if you don't like to click)</p>
  
  <pre><code>public MyContext(string connString)
      : base(connString)
  {
  
  }
  </code></pre>
  
  <p>However, when I attempt to enable migrations I receive the error: "The target context '...' is not constructible.  Add a default constructor or provide an implementation of IDbContextFactory."</p>
  
  <p>Also, when I attempt to use EF PowerTools to generate an Entity Data Model is receive the error: "A constructible type deriving from DbContext could not be found in the selected file."</p>
  
  <p>Is this a common problem?  I have a fix, but it feels like a hack.  Any one else have to deal with this before?</p>