DOC: Add a bit more explanation of bin setting in setImageArea.
ENH: Release the GIL while reading a row in grabFrame.

This allows more interactivity when acguiring images in a GUI driven
application. The other FLI functions are fast enough that it probably
isn't worth doing for them. Note that fli calls probably need to be
protected by a lock, as libfli doesn't seem set up for concurrent
operation.
ENH: Serialize device access.

The device lock (mutex) is grabbed before every access to the device.
For python threading this is normally not neccessary, as C functions are
called with the GIL held, but for grabFrame the GIL is released when
each row is read in so as to increase interactivity when used in a
threaded environment. Thus the lock prevents any other function from
accessing the device until the entire frame has been read.