Fix bug when using default get limit.

Bug:
The existing code passed 0 as the default page size limit when calling
getArtists, getAlbums or getTracks. However, this resulted in a response
from last.fm that contained the right data, but had empty strings as the
the page, perPage and totalPages attributes. These attributes then
failed to parse in ResponseBuilder, resulting in a
NumberFormatException.

To Reproduce:
Call Library.getArtists(String user, String apiKey) on a library with
less than 50 artists.

Fix:
To fix this I changed the library get methods (Library.getArtists,
Library.getAlbums and Library.getTracks) to omit the limit parameter
if limit <= 0. This results in last.fm using its default limit, which is
currently 50, and the response has the appropriate page, perPage and
totalPage attributes set, eliminating the parsing issue in
ResponseBuilder.