Integrate WebSocketLogger

[hustf]

This is the first attempt at loading a dedicated logger with WebSockets, and the added functionality would bump version number to 1.3.0 (1.2.0 added a number of 'show' methods').

The current logging implementation in Julia standard libraries can be used quite creatively (ref. Revise, Rebugger), and can possibly be fast as well. For applications with websockets, logging is especially helpful in determining the actual timing and bottlenecks between multiple threads of execution.

Logging utilities in this package has already seen some iterations and is mentioned in 23 issues. This PR refers issue #111. From inline doc:

Differences to stdlib/Logging/ConsoleLogger:

    - default timestamp on logging messages (except @info)
    - a 'shouldlog' function can be passed in. The `shouldlog_default` function filters
        on HTTP.Servers messages as well as on message_limits
    - :wslog => true flag which may be used for context-sensitive output
        from 'show' methods. This means a user can define 'show' methods
        which are used with this logger without affecting the behaviour
        defined in other modules.
    - :limited => true is included in the default IOContext. Keyword: show_limited
    - string_with_env_ws is exported for easy overloading on specific types
    - @info, @debug, @warn etc. will splat the first argument if it's a tuple arguments, e.g.

    julia> var = "a"
    "a"
    julia> @info (1, var)
    [ Info: 1a

modified: README.md WebSocketLogger and v1.3.0
new file: examples/count_with_logger.jl Logging async Websocket peers
modified: examples/minimal_server.jl Use WebSocketLogger
deleted: logutils/log_http.jl
deleted: logutils/logutils_ws.jl
new file: src/Logger/websocketlogger.jl stdlib/ConsoleLogger + /logutils.jl
modified: src/WebSockets.jl Include / export WebSocketLogger, @WSLog
modified: test/logformat.jl ConsoleLogger -> WebSocketLogger
modified: test/runtests.jl Include test_websoketlogger.jl
new file: test/test_websocketlogger.jl

[coveralls]

Coverage increased (+1.2%) to 80.72% when pulling 8d3e6c0 on logging into 64dfe1d on master.

[hustf]

@shashi, can you try comment on the below, or suggest someone with expertise on stdlib/Logging to do it?

'/examples/count_with_logger.jl' is surprisingly slow (0.5 seconds for 20 messages). I have not made a comparison as of yet. The somewhat drastic change compared to ConsoleLogger is the addition of a 'shouldlog' function field. Is that, for some reason I can't really see, bad for performance?

[hustf]

As documented in #111, there's an apparent performance improvement. Merging this.