Treat the --deps arg as a boolean, not a string

The --deps argument was being treated as a string, causing
the default 'FALSE' to thow an error.  It is now treated
as a boolean, like the --error toggle.

The R function install.packages allows deps to be either
a logical/boolean value or a string with any combination of
the elements SUGGESTS, IMPORTS, DEPENDS, etc, so this
change limits the functionality.  Trying to handle both
cases with command line arguments would probably make the
code too messy, so I just stuck with the boolean-only strategy.
Ugh, another bug in install2.r deps
The default argument should be NA, not 'FALSE'.  Consequently, we need to do the type error handling to coerce
logicals strigns to logicals and NA strings to NA.  On the down side this means we use the inconsistent-looking
notation to pass the value --deps TRUE while passing error as a boolean with the flag alone (--error).  On the
upside we can also pass strings like c('IMPORTS', 'DEPENDS').
Merge branch 'master' of https://github.com/eddelbuettel/littler
Merge pull request #5 from cboettig/master

Treat the --deps arg as a boolean, not a string [bugfix]