Add missing lxml dependency

INSTALL

@@ -5,6 +5,7 @@ Prerequisites
 -------------
 
  * Python 2.4 or later (http://www.python.org/)
+ * lxml (http://codespeak.net/lxml/)
  * trytond (http://www.tryton.org/)
 
 Installation

setup.py

@@ -10,7 +10,7 @@ major_version, minor_version, _ = info.get('version', '0.0.1').split('.', 2)
 major_version = int(major_version)
 minor_version = int(minor_version)
 
-requires = []
+requires = ['lxml']
 for dep in info.get('depends', []):
     if not re.match(r'(ir|res|workflow|webdav)(\W|$)', dep):
         requires.append('trytond_%s >= %s.%s, < %s.%s' %