Add a MainClass that tells the user they can't just run the JDBC driver

After one too many reports of
    "Failed to load Main-Class manifest attribute from postgresql-xxx.jar"
I'm submitting a dummy main-class that tells the user what they should
do instead.

The message looks like:

------------
PostgreSQL x.y JDBC4.1 (build bbbb)
Found in: jar:file:/path/to/postgresql-x.y-bbbb.jdbc41.jar!/org/postgresql/Driver.class

The PgJDBC driver is not an executable Java program.

You must install it according to the JDBC driver installation instructions for your application / container / appserver, then use it by specifying a JDBC URL of the form
    jdbc:postgresql://
or using an application specific method.

See the PgJDBC documentation: http://jdbc.postgresql.org/documentation/head/index.html

This command has had no effect.
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Merge pull request #112 from ringerc/AddMainClass

Add a MainClass that tells the user they can't just run the JDBC driver