bugfix huge buffers

Previously, the number of rows in the buffer was equal to the number of
elements in the chunk, which can be pretty big. In combination with a
big enough rowsize, at iteration time (i.e. in array.next()), a huge
array might try to be allocated. This operation sometime fails with a
MemoryError, and increases the memory footprint of the application when
it doesn't. According to gh-206, this operation was meant to satisfy the
assertion in tableextension.pyx, but chunksize is defined there as
chunkshape[0], not as chunshape.prod() (perhaps it is a bit of a
misnomer)
Add VLArray.get_row_size to the reference manual.
Compute the number of rows for the read buffer along the 'maindim' axis
Update the release notes for gh-316
Merge branch 'rowsinbuf_fix' into develop