Improve performance

[MarcoTomasRodriguez]

wget is, in some cases, faster than hget.

For example, https://storage.googleapis.com/golang/go1.6.3.darwin-amd64.pkg
wget: 25 secs
hget (1 thread): 23 secs
hget (8 threads): 26 secs

Reference: huydx/hget#18

[MarcoTomasRodriguez]

This is completely understandable because creating multiple threads has its overhead. hget with parallelism enabled is only faster when the bottleneck is in the server, not in the user. The Google servers are powerful and well optimized, so the bottleneck is probably always in the user; for these cases, it is not recommended to use parallelism, rather use a single thread.
In the future, it would be useful to let the program decide which strategy should it use.