use iobuffer instead of string for streaming files

[yeesian]

replace string concatenation in src/streaming.jl with IOBuffer

[yeesian]

Here's the performance difference when parsing OSM files:

(previously)

julia> @time nodes, highways, buildings, features = getOSMData(filename, true);
elapsed time: 627.010413888 seconds (176812171176 bytes allocated, 89.00% gc time)

(after the PR)

julia> @time nodes, highways, buildings, features = getOSMData(filename, true);
elapsed time: 85.566212155 seconds (10574148240 bytes allocated, 65.05% gc time)

[amitmurthy]

Thanks.