Check if required builtin functions are enabled

This library relies on `system` and `exec`. Solves issue #168

src/FriendlyErrors.php

@@ -3,9 +3,29 @@
 class ImageNotFoundException extends \Exception {}
 class TesseractNotFoundException extends \Exception {}
 class UnsuccessfulCommandException extends \Exception {}
+class DisabledRequiredFunctionsException extends \Exception {}
 
 class FriendlyErrors
 {
+	public static function checkDisabledFunctions()
+	{
+		$requiredFuncs = array('exec', 'system');
+		$disabledFuncs = explode(',', ini_get('disabled_functions'));
+		$disabledFuncs = array_map('trim', $disabledFuncs);
+
+		foreach($requiredFuncs as $func)
+		{
+			if (function_exists($func) && !in_array($func, $disabledFuncs))
+				continue;
+
+			$msg = "Error! TesseractOCR library requires \"$func\", ";
+			$msg.= 'but it is disabled on your system.';
+			$msg.= PHP_EOL . PHP_EOL;
+			$msg.= 'Check "disabled_functions" on your php.ini settings.';
+			throw new DisabledRequiredFunctionsException($msg);
+		}
+	}
+
 	public static function checkImagePath($image)
 	{
 		if (file_exists($image)) return;

src/TesseractOCR.php

@@ -16,6 +16,7 @@ public function __construct($image=null, $command=null)
 
 	public function run()
 	{
+		FriendlyErrors::checkDisabledFunctions();
 		FriendlyErrors::checkTesseractPresence($this->command->executable);
 		FriendlyErrors::checkImagePath($this->command->image);
 

tests/EndToEnd/FriendlyErrors.php

@@ -7,9 +7,29 @@
 use thiagoalessio\TesseractOCR\ImageNotFoundException;
 use thiagoalessio\TesseractOCR\TesseractNotFoundException;
 use thiagoalessio\TesseractOCR\UnsuccessfulCommandException;
+use thiagoalessio\TesseractOCR\DisabledRequiredFunctionsException;
 
 class FriendlyErrors extends TestCase
 {
+	public function testDisabledRequiredFunctions()
+	{
+		$originalDisabledFunctions = ini_get('disabled_functions');
+		ini_set('disabled_functions', "${originalDisabledFunctions},exec");
+
+		$expected = "Error! TesseractOCR library requires \"exec\", ";
+		$expected.= 'but it is disabled on your system.';
+		$expected.= PHP_EOL . PHP_EOL;
+		$expected.= 'Check "disabled_functions" on your php.ini settings.';
+		try {
+			(new TesseractOCR('./tests/EndToEnd/images/text.png'))->run();
+			throw new \Exception('DisabledRequiredFunctionsException not thrown');
+		} catch (DisabledRequiredFunctionsException $e) {
+			$this->assertEquals($expected, $e->getMessage());
+		}
+
+		ini_set('disabled_functions', $originalDisabledFunctions);
+	}
+
 	public function testImageNotFound()
 	{
 		$currentDir = realpath(