Fixing line_pos_iterator column numbering bug and get_current_line bug.

This is cleaned up version of https://github.com/boostorg/spirit/pull/39.

This pull request fixes following bugs.
- line_pos_iterator's column numbering bug.
- line_pos_iterator's get_current_line bug.

And add regression test for line_pos_iterator.
The `BOOST_TEST` macro is the one that provides the least information, as it simply informs whether something does not hold. I recommend you use `BOOST_TEST_EQ` instead, which would print the value of the operands when the check does not hold. This would also mean splitting the compound first check into two independent checks. That way, if this test were to fail in one of the many machines running the regression tests, we would have as much information about the failures as possible.