Fix C/C++ problems causing R CMD CHECK errors.

melt.data.frame() throws when encountering objects of type POSIXlt, and requests a conversion to the (much saner) POSIXct type.

melt.data.frame() now properly sets the OBJECT bit on value variable generated if attributes are copied (for example, when multiple POSIXct columns are concatenated to generate the value variable) (#50)

melt.data.frame() can melt data.frames containing list elements as id columns. (#49)

melt.data.frame() no longer errors when measure.vars is NULL or empty. (#46)

dcast() and acast() gain a useful error message if you use value_var intead of value.var (#16), and if value.var doesn't exist (#9). They also work better with . in specifications like . ~ . or x + y ~ .

melt.array() creates factor variables with levels in the same order as the original rownames (#19)

melt.data.frame() gains an internal Rcpp / C++ implementation, and is now many orders of magnitudes faster. It also preserves identical attributes for measure variables, and now throws a warning if they are dropped. (Thanks to Kevin Ushey)

melt.data.frame() gains a factorsAsStrings argument that controls whether factors are converted to character when melted as measure variables. This is TRUE by default for backward compatibility.

melt.array() gains a as.is argument which can be used to prevent dimnames being converted with type.convert()

recast() now returns a data frame instead of a list (#45).

Fix incompatibility with plyr 1.8

Fix evaluation bug revealed by knitr. (Fixes #18)

Fixed a bug in melt where it didn't automatically get variable names when used with tables. (Thanks to Winston Chang)

Fixed bug in melt where factors were converted to integers, instead of to characters

When the measured variable is a factor, dcast now converts it to a character rather than throwing an error. acast still returns a factor matrix. (Thanks to Brian Diggs.)

acast is now much faster, due to fixing a very slow way of naming the output. (Thanks to José Bartolomei Díaz for the bug report)

value_var argument to acast and dcast renamed to value.var to be consistent with other argument names

Order NA factor levels before (all) when creating margins

Corrected reshape citation.

melt.data.frame no longer turns characters into factors

All melt methods gain a na.rm and value.name arguments - these previously were only possessed by melt.data.frame (Fixes #5)