Add XML documentation to matrix Add methods.

Fixes warnings [#61] about missing documentation on public members.
Remove unused field.

Fixes warnings [#61] by removing an unused field.
Remove unused fields/variables.

Fixes warnings [#61] by removing some unused variables and an unused field.
Field was private so no inheritance concerns.
Add #pragma warning disable for unused field warnings.

Fixes warnings [#61] by disabling unused field warnings for two structures in
HidProtocol. These fields aren't currently used by OpenTK but the stuctures are
used in native marshalling so must match the documented structures perfectly.