Move regex check on JNumber into companion

[sirthias]

Currently the JNumber constructor always performs a regex check, even if the argument is known to never fail it (e.g. when it's called from one of the several JNumber.apply overloads).

This ticket proposes to mark the JNumber case class constructor private, add an JNumber.apply(String) overload and move the regex check into this method.

Unfortunately this fix depends on two patches to scalac (scala/scala#5730 and scala/scala#5846), which are only available as of Scala 2.11.11 (where it needs the -Xsource:2.12 flag) and 2.12.2 (no flags required) and thus won't work on Scala 2.10.

Nevertheless I think it'd be a valuable improvement that might also help to (somewhat) alleviate the concerns that @fommil has voiced in #16.

[mdedetrich]

Something similar is already being done, will see if I can integrate this. Expect an update within a couple of days

[fommil]

you can always do a hand rolled class. case class is just a boilerplate generator, afterall.

[mdedetrich]

I am going to do this in a new PR since it changes source compatibility, I am likely to just use a class, no one is going to be using the JNumber constructor directly anyways