Implements #368

Fixes #392

Typedefs that resolve to slices are not recorded in DWARF as typedefs
but instead as structs in a way that there is no way to know they
are really slices using debug/dwarf.
Using golang.org/x/debug/dwarf instead this problem is solved and
as a bonus some types are printed with a nicer names: (struct string
→ string, struct []int → []int, etc)

 Fixes #356 and #293

Additionally removes test code / html / txt / etc from vendor dir.

Use godeps to vendor dependencies and the normal go toolchain, with
GO15VENDOREXPERIMENT=1 set to properly vendor dependencies for reproducible
builds.