Add insert methods to java.lang.StringBuilder

StringBuilder is missing insert methods, I tried to make it as similar as possible to `append` methods.

Two things:
1) inserting is using `substring` instead of `splitAt` cause it's obviously more effective way if you look at the impl
2) the `removing` stuff in StringBufferTest is just renaming of a variable that had a misleading name `newBuf` - it's a builder, not buffer
Use `T : ClassTag` instead. `Manifest` is deprecated since 2.10, and does not completely work in Scala.js. This is the reason for the failures in our Continuous Integration.