#63 Add Excel-CID reader for data format

 * cid reader reads `check fields`
 * add_check skips blank cells between description and type (happens, when cells are concatenated)
 * implemented a test for reading checks of type isUnique and DistinctCount
 * moved errors from checks.py to errors.py
#66 Cleanup PEP8 issues

* checks.py

affected by checks.py refactoring:
 * interface.py
 * test_checks.py
 * test_sniff.py
Merge remote-tracking branch 'origin/htlwrn' into htlwrn