ENH: Add decimal parameter to to_numeric
Create a class / method for number parsing. This would be a generalization of the code that remove the thousands separators from numbers (see: https://github.com/cancan101/pandas/blob/1703ef44cd6b98e17c785c9120e29bbeefdefd1c/pandas/io/parsers.py#L1502).

Other items include:
1. Alternative decimal point
2. Using parenthesis for negative numbers
3. Stripping currency character

I can submit PR for this. Do people have other suggestions for this?
