Merge pull request #1 from creswick/master

moved Stream instance for lists (Text.Parsec.String to Text.Parsec.Prim)
Track current position in the 'tokens' function

Example

Before:

  Prelude Text.Parsec> parseTest (string "abcd") "abbe"
  parse error at (line 1, column 1):
  unexpected "b"
  expecting "abcd"

After:

  *Main> parseTest (string "abcd") "abbe"
  parse error at (line 1, column 3):
  unexpected "b"
  expecting "cd"

Before this patch, 'b' was reported to be found at 1:1, which is clearly not the
case. After this patch, we correctly report the location of the wrong token, and
also report missing tokens starting from that position.
Rewrite error handling
Add tests