Fix docstring of Connector::MAX_GET_URL_LENGTH

* VuFindSearch/Backend/Solr/Connector.php (MAX_GET_URL_LENGTH): Fix
docstring.
Merge branch 'master' of ssh://vufind.git.sourceforge.net/gitroot/vufind/vufind
New function: Exchange parameter array in ParamBag

* VuFindSearch/ParamBag.php (exchangeArray): New function. Exchange
  parameter array.
New class: Abstract base class of handler maps

* VuFindSearch/Backend/AbstractHandlerMap.php: New class. Abstract
  base class of handler maps.
Use internal function to set new parameter array

* VuFindSearch/ParamBag.php (exchangeArray): Use internal function
  to set new parameter array.
Remove ParamBag::params(), use ParamBag::getArrayCopy() instead

* VuFindSearch/ParamBag.php (params): Removed. Use getArrayCopy()
  instead.
Expect ParamBag as return value for HandlerMap D/A/I

* VuFindSearch/Backend/AbstractHandlerMap.php: Expect ParamBag as
  return value for D/A/I.
New class: SOLR handler map

* VuFindSearch/Backend/Solr/HandlerMap.php: New class. SOLR
  handler map.
Merge branch 'master' of ssh://vufind.git.sourceforge.net/gitroot/vufind/vufind