Require the minimal dependencies in python package

When an user installs offlineimap from PyPI using pip, the dependencies
of offlineimap are not installed automatically. See #661.

Requiring explicitly the dependencies in the setup.py adds them in the
metadata of the package so pip can install them next with offlineimap.

To avoid duplicated dependencies, requirements.txt delegates to setup.py
the listing of the minimal dependencies while also adding two more
optional dependencies.

Signed-off-by: Martin Di Paola <martinp.dipaola@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

requirements.txt

@@ -1,5 +1,5 @@
-# Requirements
-six
+# Minimal requirements defined in setup.py
+-e .
+# Extra "optional" requirements
 gssapi[kerberos]
 portalocker[cygwin]
-rfc6555

setup.py

@@ -58,6 +58,11 @@ class TestCommand(Command):
         #TODO: failfast does not seem to exist in python2.6?
         TextTestRunner(verbosity=2,failfast=True).run(suite)
 
+reqs = [
+    'six',
+    'rfc6555'
+    ]
+
 setup(name = "offlineimap",
       version = __version__,
       description = __description__,
@@ -71,6 +76,7 @@ setup(name = "offlineimap",
       scripts = ['bin/offlineimap'],
       license = __copyright__ + \
                 ", Licensed under the GPL version 2",
-      cmdclass = { 'test': TestCommand}
+      cmdclass = { 'test': TestCommand},
+      install_requires = reqs
 )