Add coverage checks and coveralls integration (#64)

* Add coverage checks  and coveralls integration
* Avoid problem with zc.recipe.egg
* Add coverage config.

.coveragerc

@@ -0,0 +1,11 @@
+[run]
+source = z3c.form
+
+[report]
+precision = 2
+exclude_lines =
+    pragma: no cover
+    if __name__ == '__main__':
+    raise NotImplementedError
+    self.fail
+    raise AssertionError

.travis.yml

@@ -1,16 +1,18 @@
 sudo: false
-cache:
-  directories:
-    - .tox
 language: python
 python:
     - 2.7
     - 3.3
     - 3.4
     - 3.5
 install:
-    - travis_retry pip install tox
+    - pip install -U pip setuptools
+    - pip install -U coverage coveralls zope.testrunner
+    - pip install -U -e .[test]
 script:
-    - tox -e py
+    - coverage run -m zope.testrunner --test-path=src
+after_success:
+    - coveralls
 notifications:
     email: false
+cache: pip