this allows printing the actual method signature of the function in the call stack

both profiling and backtrace printing have been extended to show this
full linfo signature (when available)

Tests and documentation for stacktraces.
Updated NEWS.md with stacktraces functionality.
Refactored, doctests, show_stacktrace is now show.
Added doctests to the manual.
Removed show_stacktrace in favour of show(x::StackFrame).
Removed format_stacktrace and format_stackframe.
Additional test cases (which currently fail due to difficult to track down line number issues)
Test for invalid frame. Removed failing Win32 test.
Reintroduced UNKNOWN constant stack frame from profile.jl.
Added test case for looking up an invalid stack frame.
Selectively removed some tests that consistently fail in Appveyor's 32-bit Windows LVM.