add driver client

[smola]

Add driver client to be used by the server.

It should implement, at least:

ParseUAST(req *protocol.ParseUASTRequest) (*protocol.ParseUASTResponse, error)

The rest of it should be something along the lines of:

type Driver struct{}
func ExecDriver(r *runtime.Runtime, desc *DriverDescriptor) (*Driver, error)
func (d *Driver) ParseUAST(req *protocol.ParseUASTRequest) (*protocol.ParseUASTResponse, error)
func (d *Driver) Close() error