updating to re-written corpus.R

needs documentation
re-written version. needs docs
now uses new accessor and tokenize
re-written, needs docs.
re-written version. needs docs
added indexer and examples from re-write branch
cleaning up
checking tokens attribute
work on dfm
 changes to gitignore
Delete .Rbuildignore
removign unncessary files
Merge branch 'dev' of https://github.com/kbenoit/quanteda into dev
removing rproj from github
removing check files
Adding some tutorials files
editing examples
cleaning
UK 2010 manifestos immigration sections, in ASCII text.  (Encoding bit is set to "unknown" since this is the rule for ASCII.)
Merge branch 'dev' of https://github.com/kbenoit/quanteda into dev