floating point errors due to implicit float conversion

[mileslucas]

CC #65

The following code currently fails because AstroPeriod is constructed implicitly using Float64

julia> typemax(Int64)*seconds
ERROR: InexactError: trunc(Int64, 9.223372036854776e18)
Stacktrace:
 [1] trunc
   @ ./float.jl:716 [inlined]
 [2] floor
   @ ./float.jl:294 [inlined]
 [3] AstroPeriod
   @ ~/dev/julia/AstroTime/src/Periods.jl:97 [inlined]
 [4] *(dt::Int64, unit::AstroTime.Periods.Second)
   @ AstroTime.Periods ~/dev/julia/AstroTime/src/Periods.jl:152
 [5] top-level scope
   @ REPL[27]:1

I'm sure there's probably a simple fix using oftype, I don't have time at the moment to dig into it though.