Provide a global tolerance when comparing for equality

This PR tries to solve #18.

As proposed it implements an optional parameter set along the following example:
```
{
  tolerance : 0.05,
  globalTolerance : true
}
```

Following those changes `imagediff.equal(a, b, 128)` would be expressed through `imagediff.equal(a, b, {tolerance: 0.5})` in the future (however the old API remains intact).
A global tolerance can be given through adding `globalTolerance: true` to the option object.