Vmprof

* register_virtual_function cannot exhaust the buffers any more
* do not write over the boundary of the buffer that contains the stack trace
* print better warnings when the program run is too short
* add vmprof.is_enabled/vmprof.get_profile_path
* Pull request (only needed symbols), do not log function names that are not in the profile
* Better descriptor checks (e.g. cannot give a non writable fd to vmprof.enable)
* Warn user if vmprof file is bigger than 100 MB and uploaded to vmprof.com (might be rejected)
* Source distribution includes dwarf.def and other files required to build
* osx: dlopen+dlsym not used for libunwind (use standard way to link to libunwind)

- pypy support to native profiling
- dumping native symbols is not implemented in C (to not maintain a C and a Python version)
- sample_stack_now has an optional paramter 'skip' which skips a certain amount of native frames
- win64 + cpython 3.6 compilation issue solved
- dynamic loading of libunwind symbols (to support pypy)
- added sqlite sample file
- do not overwrite base_headers for http requests if authorization is provided

Serveral enhancements
- 116 vmprof hangs when multiple threads frequently release/acquire the GIl
- 118 vmprofshow breaks on multiline dictionary comprehensions
- PyPy jit log issues

- Native profiling (C stack) is included in the profile (using libunwind)
for Linux & Mac
- Windows 64bit support (no native profiling)
- The platform that reads the profile can be different from the platform
that generated it
- vmprof.com updates to the flamegraph and general style changes
- Documentation updates

- Invoking `... -m vmprof --jitlog ...` did not upload source snippets to the remote server