RTree links wrong shared object on linux

RTree depends on libspatialindex_c.so, which is only development symlink to actual library and should be used only during compile time. This requires that dev packages of libspatialindex are also installed for RTree to work. Bugfix should be simple:

Following line: https://github.com/Toblerity/rtree/blob/master/rtree/core.py#L105
*lib_name = 'libspatialindex_c.so'*
should be replaced with
*lib_name = ctypes.find_library('spatialindex_c')*
which then selects correct name (libspatialindex_c.so.1), at least on linux.