Fix : when object is not the name of an existing data.frame

[JulienBlasco]

toxlsx() no longer fails when the object argument is the result of a computation

• replace df with df_name in loops in order to clarify that it is a string
• remove get(df_name) and use original object instead

Solves #18

[JulienBlasco]

Added a new commit 11d6156 that should fix #37

When object is a list, the output names are now defined according to three possible scenarios:

1. object is a named list, then those names are used
2. object is a call to the list() function, then the arguments of list() are used
3. other cases: generic names ("Table 1", "Table 2"...) are used