Fix for failing 'focus.application.node' seting

Test failure:
Traceback (most recent call last):
File "/home/travis/build/roignac/dogtail/tests/gtkdemotest.py", line 15, in setUp
  self.pid = dogtail.utils.run('gtk3-demo')
File "/home/travis/build/roignac/dogtail/dogtail/utils.py", line 118, in run
  focus.application.node = child
File "/home/travis/build/roignac/dogtail/dogtail/procedural.py", line 57, in __setattr__
  self.__class__.__dict__[name] = value
TypeError: 'dictproxy' object does not support item assignment
Fix for test_predicates_generic_by_label: generate a better script method call for labels

Fixes the following exception:
Traceback (most recent call last):
File "/home/travis/build/roignac/dogtail/tests/test_predicate.py", line 160, in test_predicates_generic_by_label
  False), u'child(label="dummy name 1", recursive=False)')
  AssertionError: u'child(label=\'"dummy name 1"\', recursive=False)' != u'child(label="dummy name 1", recursive=False)'
  - child(label='"dummy name 1"', recursive=False)
 ? - -
 + child(label="dummy name 1", recursive=False)
Trap GErrors in findAllDescendants and retry if it occurs

This fixes crashes in findChildren if elements are being added/removed quickly:
GError: Method "Get" with signature "ss" on interface "org.freedesktop.DBus.Properties" could not be processed as object /org/a11y/atspi/accessible/2310 does not exist
Run tests on travis and report to coveralls