add adehabitatLT datasets

[nignatiadis]

As discussed in #22. Turns out that the adehabitatLT package provides a function (ld) for converting those ltraj objects to data frames (basically adds an ID column, as I wanted to do, and a burst column), so I used that.

The datasets function was still using String notation to index the DataFrame, so I changed that to a Symbol.

[johnmyleswhite]

Looks good to me. Just to check: you've added the ID columns to each table and then added the modified tables?

[nignatiadis]

I did the following:

• If the dataset was saved as a data frame, I just exported the data frame (after changing colnames to CamelCase).
• If the dataset was saved as a ltraj object (the one I mention in issue #22), then I used the "ld" function. From the documentation: "The two functions ld and dl are useful to quickly convert objects of class ltraj from and to dataframes." and "The function ld concatenates all bursts in an object of class ltraj, adds two columns named id and burst, and, when it is present, also adds the variables in the infolocs component." So in most cases, the two columns "Burst" and "ID" were added.
• The "buffalo" dataset is a list which includes a habitat map (SpatialPixelsDataFrame) and a ltraj object. From these, I only exported the ltraj object (similar as above), because the size of the habitat map was >4 Mb.

[johnmyleswhite]

Great. That sounds like the right approach.