Bugs corrected in the R interface

• Some small changes to make R CMD check clean
• Fixed a bug in graph.incidence, the directed and mode arguments were not handled correctly
• Betweenness and edge betweenness functions work for graphs with many shortest paths now (up to the limit of long long int)
• When compiling the package, the configure script fails if there is no C compiler available
• igraph.from.graphNEL creates the right number of loop edges now
• Fixed a bug in bipartite.projection() that caused occasional crashes on some systems