Add jira, megaplan and activecollab deps as extras

This uses the extras_require value in setup.py. For instance , if you
want to use bugwarrior with jira you need to install it with:

  pip install bugwarrior[jira]

.travis.yml

@@ -17,7 +17,7 @@ before_install:
   - pip install mock
   - pip install unittest2
   - cd $TRAVIS_BUILD_DIR
-install: python setup.py install
+install: pip install .[jira,megaplan,activecollab]
 script: nosetests -w tests
 notifications:
     email: true

setup.py

@@ -44,11 +44,12 @@
           "lockfile>=0.9.1",
           "click",
           "pyxdg",
-          "jira>=0.22",
-          "megaplan>=1.4",
-          "pypandoc",
-          "pyac",
       ],
+      extras_require=dict(
+          jira=["jira>=0.22"],
+          megaplan=["megaplan>=1.4"],
+          activecollab=["pypandoc", "pyac"]
+      ),
       tests_require=[
           "Mock",
           "unittest2",