Added optional argument to contains_header to skip multiple header lines

Some banks add initial lines to the csv file with for example your
starting balance. This option allows one to skip these lines
Only set contain_headers if not nil, otherwise set to 1

Fixes bug since nil.to_i is 0
Merge pull request #22 from BlackEdder/skip_lines2

Added optional argument to contains_header to skip multiple header lines
Merge branch 'upstream' into nationwide
New minimal spec to test nationwide conversion