Merge pull request #3 from briatte/extradoc

extra documentation and little fixes
dependencies
ridiculous stopifnot fix
tentative qogdata/ts-but-not-ts bugfix

still could be improved at xtset stage (cname might not fall in the right place if ccode is not present)
Merge pull request #5 from briatte/extradoc

attempt to get xtdata and xtcheck right
Merge pull request #6 from briatte/master

update extradoc to latest bugfixes