Added dub.json and renamed assertion file for compatibility

The library compiles fine in dub using the file and a minor fix on assertion.d's naming - the filename must correspond to the module name.

I'm getting errors though on fixed.d unit test, the constructor doesn't seem to be addressable during compile-time 

	eris\integer\fixed.d(458): Error: template eris.integer.fixed.FixedInt!(128u, fa
	lse, false).FixedInt.__ctor cannot deduce function from argument types !()(uint[
	]), candidates are:
	eris\integer\fixed.d(86):        eris.integer.fixed.FixedInt!(128u, false, false
	).FixedInt.__ctor(T : uint[])(const T digits)
	eris\integer\fixed.d(125):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T)(const T num) if (__traits(isIntegral, T) && !__traits(isUn
	signed, T))
	eris\integer\fixed.d(138):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T : bool)(const T value)
	eris\integer\fixed.d(205):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T)(const T num) if (__traits(isIntegral, T) && __traits(isUns
	igned, T))
	eris\integer\fixed.d(268):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T)(const T that) if (__traits(hasMember, that, "IS_FIXED_INTE
	GER"))
	eris\integer\fixed.d(292):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T : FixedInt!(128u, false, false))(const T that)
	eris\integer\fixed.d(332):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T : ubyte[])(T buffer)
	eris\integer\fixed.d(623):        eris.integer.fixed.FixedInt!(128u, false, fals
	e).FixedInt.__ctor(T : string)(const T str)