Adds Device.FFCaps method. It returns the number of
simulteneous Force Feedback effects supported by a
device, as well as the a bitset defining the
type of effects.
Renames Device.FFCaps to Device.ForceFeedbackCaps.

Adds Device.Supports() method. It accepts a variable
set of EvXXX constants and it returns true only if
the device supports all of these devices.

This is a helper method which can be used to quickly
test if a given device meets our needs.
Amends README with a note about lacking Force Feedback API bits.
Refactors some code in input_h.go and handles processing
of some more FF effect types.
Refactors some code to simplify the API.
Adds querying of Force feedback effects.
Adds capacity to upload, modify and toggle playback of
force feedback effects.
Fixes example programs to reflect changes in Device API.
Adds examples/forcefeedback to demonstrate how to use the
various FF related API components.

This allows one to query for Force Feedback capabilities,
create and upload new effects and toggle their play state.