Refactored subcommand tests.
Change assertion order for assert_subcommand_parsing
Pep8
Moved Sandbox in test_helpers
Renamed test_helpers.py to helpers.py
Created YodaTestHelper class

This class contains custom assertions for yoda's test suite.
Its goal is to be the base test case for all yoda's tests, and provides
useful assertions to simplify the writing and reading tests.
Created AdapterTestHelper class

This class contains common code for yoda's adapters test cases.
Pep8
Removed useless import
Use YodaTestHelper for some test cases.

YodaTestHelper create and destroy sandbox on setup/teardown. Test cases
that needs sandbox use this helper instead unittest.TestCase.
Fixed duplicate code
Removed AdapterTestHelper.adapter_class attribute.
Merge branch 'tests-refactoring'

Conflicts:
	tests/adapter/git_test.py
	tests/adapter/svn_test.py