Add md5 and sha1 checksum to General.

GeneralPlugin now returns as follows:

<pre>Analysing build.xml...
array(2) {
  ["BoxUK\General"]=>
  array(7) {
    ["errors"]=>
    array(0) {
    }
    ["extension"]=>
    string(3) "xml"
    ["type"]=>
    NULL
    ["mimeType"]=>
    string(15) "application/xml"
    ["fileSizeInBytes"]=>
    int(1280)
    ["md5Checksum"]=>
    string(32) "8bc143c0314e5f170a7942f2291329eb"
    ["sha1Checksum"]=>
    string(40) "f5503a09c219801fd3c684554f00cea386e5c6fe"
  }
  ["BoxUK\PlainText"]=>
  array(4) {
    ["errors"]=>
    array(0) {
    }
    ["lines"]=>
    int(34)
    ["characters"]=>
    int(1280)
    ["words"]=>
    int(128)
  }
}</pre>