Added dub.json and renamed assertion file for compatibility

[etcimon]

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 with dmd 2.066, 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)