Multiple apps: Default binary path by platform

Use Python's platform library to detect the operating system and
use different default paths for the binary dependencies.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/document_parsing/settings.py

@@ -4,6 +4,8 @@ from django.utils.translation import ugettext_lazy as _
 
 from mayan.apps.smart_settings import Namespace
 
+from .literals import DEFAULT_PDFTOTEXT_PATH
+
 namespace = Namespace(name='document_parsing', label=_('Document parsing'))
 
 setting_auto_parsing = namespace.add_setting(
@@ -14,7 +16,7 @@ setting_auto_parsing = namespace.add_setting(
 )
 setting_pdftotext_path = namespace.add_setting(
     global_name='DOCUMENT_PARSING_PDFTOTEXT_PATH',
-    default='/usr/bin/pdftotext',
+    default=DEFAULT_PDFTOTEXT_PATH,
     help_text=_(
         'File path to poppler\'s pdftotext program used to extract text '
         'from PDF files.'