support tables with __call metamethod in handlers

[matthargett]

The library currently checks for:

	assert(
		successHandler == nil or type(successHandler) == "function",
		string.format(ERROR_NON_FUNCTION, "Promise:andThen")
	)
	assert(
		failureHandler == nil or type(failureHandler) == "function",
		string.format(ERROR_NON_FUNCTION, "Promise:andThen")
	)

which will fail for callable tables that use the __call metamethod, so this code fails the assertion above:

		local wrapped = {}
		setmetatable(wrapped, {
			__call = _wrapped,
		})
		wrapped.cancel = _cancel

                Promise.new():andThen(wrapped, wrapped)

a helper method that could be used in the library might look like:

local function isCallable(value)
  if typeof(value) == "function" then
    return true
  end
  if typeof(value) == "table" then
    local mt = getmetatable(value)
    if mt and rawget(mt, "__call") then
      return true
    end
  end
  return false
end

[evaera]

Supporting callable tables sounds good to me. We should make sure that we update this behavior everywhere we check for things that are functions.