implement summary with unlimited length

this commit fixes middleman/middleman#1074

If the summary_length is set to any value smaller than 0
and a summary_separtor is defined than the summary is
generated in case of
a) no separtor found in article: whole article (summary = article)
b) separator found in article: article until the occurence of the separator
Merge pull request #174 from rriemann/implement-unlimited-summary-length

implement summary with unlimited length