1. Shrink DoubleBufferReader's buffer size to 3.
2. Add BatchReader an option to discard leftover instances.
3. Fix a MultiPassReader bug on pass count.

1. Make PyReader and RandomDataGenerator inherited from FileReader.
2. Remove the memeber variable 'dims_' and realated checks in FileReader.

1. Add a new attribute named 'startuo_seed' to RandomCropOp. If the input
'Seed' is not initialized, the 'startup_seed' will be used to replace
it.

2. Refine CustomReader. Add a member variable 'scope_' to it. The
'scope_' will act as the global scope of preprocesing, making it
possiable to save something cross batches.

* less_than
* cumsum
* multiplex
* open_recordio_file