close #262; close #263
close #261
closes #257
closes #258 as addfieldusingcontext
closes #251; closes #265
closes #255
closes #266
closes #252
closes #177
closes #249; simplify natural joins
closes #264
closes #246
closes #247 by adding "petl" class to <table> element
#267 refactor petl.transform into submodules
#267 factor out petl.transform.headers
#267 factor out petl.transform.regex
#267 factor out petl.transform.reshape
#267 factor out petl.transform.maps
#267 factor out petl.transform.unpacks
#267 factor out petl.transform.dedup and petl.transform.setops