Added dependencies to plugins.

Now plugins can require that other plugins be loaded first by adding the
class attribute [PluginDependencies(typeof(<required type>))].  For
example, a plugin  that is dependent on Oda.Sql would look like
[PluginDependencies(typeof(Oda.Sql))].  The attribute class
PluginDependencies has two overloads allowing for different ways of
declaring one or more dependencies.
Added plugin dependency.  Changed class name.

Added dependency attribute [PluginDependencies(typeof(Sql))] to the
AuthenticationPlugin class.  Now the Oda.Sql plugin will load prior to
this plugin loading.  This follows changes to the core assembly in
commit 29a2291.  Also, changed the plugin class name from SessionInit to
AuthenticationPlugin.  Now it's 72% less stupid sounding.