add component/nltk.

* patch nltk egg so that the default data location is (buildout_directory)/parts/nltk-data without explicitly specifing by NLTK_DATA environment variable.
* download collection can be overridden like 'collection = aaa bbb ccc'.

/ref http://community.slapos.org/forum/NLTK-Buildout--Download-and-install-nltk_data

/cc @klaus, @Kreisel 

/reviewed-on !58

component/nltk/buildout.cfg

+[nltk]
+recipe = zc.recipe.egg
+eggs = nltk
+interpreter = py-nltk
+nltk-patches = ${:_profile_base_location_}/nltk-data-path.patch#8719f78f05ac70752b4e64fe086e36e2
+nltk-patch-options = -p1
+
+[nltk-data]
+recipe = plone.recipe.command
+location = ${buildout:parts-directory}/${:_buildout_section_name_}
+# You can override the collection like 'collection = aaa bbb ccc'
+collection = all
+command = ${buildout:bin-directory}/${nltk:interpreter} -m nltk.downloader -- -d ${:location} ${:collection}

component/nltk/nltk-data-path.patch

+diff -ur nltk-3.2.orig/nltk/data.py nltk-3.2/nltk/data.py
+--- nltk-3.2.orig/nltk/data.py	2016-03-02 22:56:32.000000000 +0100
++++ nltk-3.2/nltk/data.py	2016-03-13 18:00:32.192142493 +0100
+@@ -77,6 +77,9 @@
+ if 'APPENGINE_RUNTIME' not in os.environ and os.path.expanduser('~/') != '~/':
+     path.append(os.path.expanduser(str('~/nltk_data')))
+ 
++path += [os.path.join(os.path.dirname(os.path.abspath(__file__)),
++                      '..', '..', '..', 'parts', 'nltk-data')]
++
+ if sys.platform.startswith('win'):
+     # Common locations on Windows:
+     path += [