added ability to set the number of logfile lines displayed on errors (number or 'ALL')

According to:

https://github.com/rock-core/autobuild/issues/6
Use Float::INFINITY as the "all lines" value. Moreover, in general, nil is the default "magic value" in Ruby.

Use accessors instead of referring to instance variables directly (requires defining displayed_error_line_count as attr_accessor instead of only attr_writer)