Feature: Auto-queue SQL if busy, replay as able.

This is a simple (naïve?) FIFO queue implementation. It must be enabled using the `useQueue: true` configuration option which is passed to the new Connection object.

Several of my production projects try to use the SQL connection often, which occasionally results in `execSql()` being called while the Connection is busy. In the majority of cases I do not need nor have a desire to maintain multiple connections to the SQL server; it is much more useful to me to automatically store the SQL request and execute it as the connection is available, rather than trying to handle the error and re-try outside of the SQL library (tedious).

This patch provides queue support capability for Tedious.
It is only lightly tested so far, but it appears to work in development.

A simple (CoffeeScript) test file is included below:
```coffee
tedious = require('tedious')
util = require('util')

sqlconfig =
  userName: 'USER',
  password: 'PASSWORD',
  server: 'SERVER GOES HERE'
  options:
    useQueue: true
    debug: true

tds = {}
tds.conn = new tedious.Connection(sqlconfig);
tds.conn.on 'debug', (thing) ->
  console.log util.inspect(thing, {depth: null})

tds.conn.on 'connect', (err) ->
  if err?
    console.error "tedious.Connection: ERROR: #{err}"
    process.exit(1)
    return

  console.info "Connected to database at #{sqlconfig.server} as '#{sqlconfig.userName}'."
  setImmediate(fuzz)
  setImmediate(find)

tds.conn.on 'errorMessage', (err) ->
  if err?
    console.error "tedious.Connection: ERROR: #{util.inspect(err, {depth: null})}"
    process.exit(1)
    return

tds.conn.on 'end', ->
  console.error "tedious.Connection: ERROR: Connection closed."
  process.exit(1)
  return

tds.req = tedious.Request
tds.TYPES = tedious.TYPES

tally = 0

fuzz = ->
  query = 'INSERT INTO test_table VALUES (@q)'
  sql = new tds.req query, (err, rowCount) ->
    if err?
       console.error("ERROR: #{err}")
       #return

    console.log "- fin fuzz #{tally}"
    tally++
    setImmediate(fuzz)


  sql.addParameter('q', tds.TYPES.VarChar, '1.#{tally}')

  console.log "- start fuzz #{tally}"
  tds.conn.execSql(sql)


find = ->
  query = 'SELECT * FROM test_table'
  sql = new tds.req query, (err, rowCount) ->
    if err?
       console.error("ERROR: #{err}")
       #return

    console.log "- fin find #{tally}"
    tally++
    setImmediate(find)


  console.log "- start find #{tally}"
  tds.conn.execSql(sql)

```