enable continuous integration via craigcitro/r-travis

also:

- ignore .Rhistory file created by RStudio
- bump version, add heading for new version number
New parameter .id for ldply() to specify name of index column (default: .id); fixes # 140; fixes # 107
New parameter .id for rdply() to specify name of index column (default: .n); fixes # 142
update Rd files
Generate .id column as factor in ldply(); fixes # 137
add NEWS