Add support for command variable

- Readd support for command variable to support non-standard
  installations
- Fix issue #157
Add additional tests for custom ctag commands

- Add additional tests to support new `cmd` parameter
- Insert SetUp definition to ensure `ctags_on_path` test run before all
  other tests
- Remove commented-out, invalid tests
- Minor formatting
Validate ctags.py against PEP8

- Reorder functions to suit call order
- Remove unused functions and imports
- General tidy up to pass PEP8 validation
Validate ctagsplugin.py against PEP8

- Reorder functions to suit call order
- Remove unused functions and imports
- General tidy up to pass PEP8 validation
General rework

- Add documentation for multiple path related functions
- Rename variables for same to improve readability
- Remove/Rewrite unneccessary or overcomplicated code
Modify directory-access functions

- Make functions more generic
- Add documentation for functions
Further rework

- Make file access functions more generic
- Group functions together
- Small additional fixes
Further rework

- Make file access functions more generic
- Group functions together
- Small additional fixes
Add minor fixes in ctagsplugin.py

- Add missing codecs import
- Small pep8 errors
Fix issue #93

- Generate error message when ctags executable not found