WIP: rebasing pyutils#334: implementation

line_profiler/_line_profiler.pyx
    <General>
        - Updated `extern` C-code blocks to make use of the
          `Python_wrapper.h` and `c_trace_callbacks.*` files
        - Line-wrapped some docstrings and comments
        - Updated some docstrings to be more `sphinx`-friendly
    disable_line_events()
        New wrapper for frame-local trace functions so that they set
        `FrameType.f_trace_lines` to false without messing with line
        profiling
    ThreadState
        New helper class for keeping track of the active profilers and
        trace callback on each thread
    LineProfiler
        __doc__
            Updated
        _[all_]thread_states
            Supersedes the previous private (class) attributes
            `_[all_]active_instances`
        __init__()
            Added optional argument `wrap_trace`
        enable(), diable()
            Refactored to also handle trace callbacks (using
            `ThreadState._handle_{enable,disable}_event()`)
    python_trace_callback()
        - Now taking a `ThreadState` object instead of a collection of
          profilers
        - Added hook to call other callbacks

Author: TTsangSC