New option to pass the desired Encoding to SeparatedValueParser and FixedLengthParser

Hi

I'm using your great library and I discovered some issue.

I did the following in this branch:

Tests were failing if culture was different than yours 
=> All tests are now culture independent

The StreamReader in FixedLengthParser and SeparatedValueParser used default behavior of guessing how the incoming text is encoded, but the guess almost everytime is UTF8, this doesn't work out very well here in Europe as for example we get flat files encoded with Code Page 1252.
=> I added to both Methods and their respective overloads the optional parameter Encoding, so that you can choose with which Encoding you want to read the text. I added also some tests

May I ask you to take a look into this and if you agree merge it to the master ?

Many Thanks!