====== checkAvailability ======
Checks the availability of the specified page. Terminates performing of the task, if the page was unavailable.
===== Syntax =====
<code javascript>checkAvailability = function(aPage, aExpectedIP, aIncludedText, aUserAgent);</code>
<code javascript>checkAvailability = function(aCheckItems);</code>
==== Parameters ====
  * **//aPage//** 
    * //functions:// verified page
  * **//aExpectedIP//** 
    * //functions:// supposed IP host on which the page is situated
  * **//aIncludedText//** 
    * //functions:// text contained in the source code of the checked page
  * **//aUserAgent//** 
    * //functions:// User-agent, which will be specified when loading the source code of the checked page
  * **//aCheckItem//** 
    * //functions:// record containing the information required to verify the page
  * **//aCheckItems//** 
     * //functions:// array of records to check multiple pages
===== Notes =====
  * To test the page goes through the following algorithm:
    - verified the availability of the host on which the page is situated. If the host is available, then
    - if IP host coincides with aExpectedIP or aExpectedIP and equals '0 .0.0.0 ', then
    - if aIncludedText not empty, the source code of the page is loaded using aUserAgent (redirects are not supported!). If aUserAgent empty, then used User-agent, installed for your browser. If the source code of the page contains aIncludedText or aIncludedText - empty string, then
    - test passes, the script continues to execute
  * If the test fails, the script will stop execution
  * The procedure may be performed only once, and only being the first method that  performed by a script. If there were performed  other methods before, the software will ignore CheckAvailability.
  * If you check the array pages, will be checked only the first five elements of array
===== Examples =====
  - <code javascript>checkAvailability('habrahabr.ru');</code>
  - <code javascript>checkAvailability('habrahabr.ru', '178.248.233.33');</code>
  - <code javascript>checkAvailability('habrahabr.ru', '178.248.233.33', 'habr');</code>
  - <code javascript>checkAvailability([
  {"page":"habrahabr.ru","expectedIP":"178.248.233.33","includedTextt":"habr"},
  {"page":"liveinternet.ru","includedText":"stat","userAgent":"my ua"}
]);</code>