Use Base.(at)mathconst, add sintau, costau, modtau

It's not actually necessary to define `@mathconst`--you just have to write the last term as an expression.  

In this PR, I chose to write it as `(2*big(pi))`, but you could define it directly as 

```julia
BigFloat("6.283185307179586476925286766559005768394338798750211641949889184615632812572417997256069650684234135964296173026564613294187689219101164463450718816256")
```

which is (almost) how you wrote it in your current version, except that the quotes are necessary. Without the quotes, the number is interpreted as a `Float64` and loses most of the precision.  

(Try `tau - 2*big(pi)` with your current version.)
I know, merging looking through it now. I'll merge soon. Thanks again!


On Mon, Jul 7, 2014 at 11:03 AM, Kevin Squire <notifications@github.com>
wrote:

> In src/Tau.jl:
>
> >
> >  # Use overridden macro definition to define conversion methods for tau
> > -@math_const τ        6.28318530717958647692  tau
>
> Just to check, are you asking for the patch? If so, it's included
> here--you just need to merge this pull request.
>
> My apologies if that's not a question or I'm not understanding!
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Aerlinger/Tau.jl/pull/4/files#r14611921>.
>