Added functionality for BioPac MP150 system

Useful for research with the MP150 ('squeezies'), which can be used to measure the maximum voluntary contraction (MVC). Depends on the mpdev DLL produced by BioPac (place it in the same directory as libmpdev, or make sure that it is installed on your system).
improved coding elegance in BioPac MP150 class

Fixed double sample time bug, and generally improved coding smoothness.
BioPac MP150 update

Added option to record to NumPy array buffer.
Improved DLL finding: Now the library first looks in the current directory, and if it cannot find mpdev.dll there it will look in the directory of libmpdev.py.
BioPac MP150 mpdev update
updated README
fixed post-calibration bug
Merge remote-tracking branch 'upstream/master'