provide method to operate DataInput instance

[ilaipi]

halo~
I recently done a job for analysis show2003.dbf & sjshq.dbfuse this library.
In show2003, the first row with time info is marked as deleted, so I can't get the record by nextReord().
Even in the first row, some fields' length isn't the same with the define in the header.

So I need read the time info custom by :

    /**
     * for custom user, only when you understand the file content<br/>
     * call dataInput.skip(int n)
     * @param length
     */
    skipBytes(int length)
    /**
     * for custom user, only when you understand the file content<br/>
     * call dataInput.readFully(byte[] b)
     * @param length
     */
    readBytes(int length)

And then, I also need the other "deleted data" in show2003,
So I also add a method:

    /**
     * Reads and returns the next row in the Dbf stream<br/>
     * a deleted row start with "*" <br/>
     * a normal row start with " " <br/>
     * so we just ignore the first byte
     * @return The next row as an Object array.
     */
    nextRecordIgnoreDelete()

I hope this can help to others!