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/literals.py

@@ -0,0 +1,8 @@
+from __future__ import unicode_literals
+
+import platform
+
+if platform.system() == 'OpenBSD':
+    DEFAULT_PDFTOTEXT_PATH = '/usr/local/bin/pdftotext'
+else:
+    DEFAULT_PDFTOTEXT_PATH = '/usr/bin/pdftotext'