implement Parsimmon.index to get the current index

believe it or not, this still honors the monad laws :).
implement ::mark()

for the common task of marking the beginning and end indices of an
expression.  This would be better implemented with `seq`, and can be
rewritten once that's merged.
add methods for compatibility with fantasyland

https://github.com/fantasyland/fantasy-land
Merge pull request #7 from jayferd/feature.index

Feature.index
document the fantasyland compatibility
document index and mark in the README
add the fantasyland logo
Merge branch 'feature.fantasyland'

Conflicts:
	src/parsimmon.js
Merge branch 'master' into feature.seq

Conflicts:
	src/parsimmon.js
implement mark and ap with seq