add buffer for BDF files

This PR is to address a concern brought up on the mne listserv earlier this year. Since bdf files are stored as 24-bit, and numpy doesn't support this data format, this module currently copies the data twice for conversion. If you try to preload a bdf file, it may tax the computer memory given the size of the file. To alleviate this problem, I added a small subroutine that buffers the data in 10s increment to do the conversion. This should make loading these files easier.