This branch adds a close_abort() method to the WiFiClient class.

How it works, what it does:
Calling `close_abort()` will close and abort the client connection it
is invoked on and, as a result, free its resources (i.e. memory).

**WARNING:** aborting connections without a good reason violates the
TCP protocol, because a closed connection would normally need to
spend some time in `TIME_WAIT` state before its resources are freed.

Usage example:
    WiFiClient client;		// set up a client

    { /* do things with your client */ }

    client.stop_abort()		// when you're done,abort the
				// connection if you must

Why it's useful:
1. Give programmers a way to shut down connections immediately if
   need be. The underlying `tcp.c` file has an abort function, but
   this has not been directly accessible via the `WiFiClient`
   class until now.

2. There are a number of reported issues for the repository
   addressing the heap corruption that can result from trying to
   retain too many connections in `TIME_WAIT` state (most notably:
   esp8266#230, esp8266#1070, esp8266#1923). Although the warning above holds, there may be
   circumstances where this isn't very important. For example an ESP8266
   running in AP mode hosting a page, which requests a new
   connection every second via an AJAX script to  monitor
   a sensor/button/etc. continusously.

Currently existing alternative approach:
When building a project, defining the
`-D MEMP_NUM_TCP_PCB_TIME_WAIT=5`compiler directive will limit the
maximum number of clients allowed to stay in TIME_WAIT state. `5` is
the default, lower it as necessary. See reference
[here](https://github.com/esp8266/Arduino/blob/master/tools/sdk/lwip/include/lwipopts.h#L263)

Thanks:
Thank you to @me-no-dev, @everslick and @Palatis for bringing the `
MEMP_NUM_TCP_PCB_TIME_WAIT` option to my attention.