Use int_vector_buffer for wt_int construction

also fixed a bug in ram_filebuf (should fix issue #140)
Set buffer size to 10MiB
Merge pull request #160 from simongog/remove_temp_write_read_buffer

Use int_vector_buffer for wt_int construction
Improved memory peak during wt_gmr_2 construction
Merge branch 'master' of https://github.com/simongog/sdsl-lite into rank_select_for_int_alphabets
Simplified tmp_file
Reduced memory peak during construction