Re-work the TShark finder

Proposing a change to the behavior of the TShark finding mechanism.

__Old way__: User is expected to supply some paths to search for TShark in `config.ini`. Some sensible defaults are provided in that file.

__Proposed way__: User is expected to supply _the_ path for TShark in `config.ini`. If they don't then some sensible defaults are searched.

This fixes a few potential issues:
(1) The current default `config.ini` assumes Windows is installed under `C:`
(2) The current configuration parser doesn't allow for paths with commas
(3) `setup.py` misses `config.ini` (this is [issue 18](https://github.com/KimiNewt/pyshark/issues/18))

(1) and (2) are admittedly uncommon, but (3) is probably a legitimate bug to fix. If you'd rather keep the current behavior I can submit a separate patch for (3) only.

I tested (on Windows) 32-bit Python 2.7 with 64-bit Wireshark and 64-bit Python 3.4 with 64-bit Wireshark. On Linux the default search behavior should be unchanged.