use net-http-persistent with :persistent option

* refactors http client from the client itself
* two http backends: net/http annd net-http-persistent
* added test for long running connections

and benchmark results:

ruby -v: ruby 2.0.0p451 (2014-02-24 revision 45167) [x86_64-darwin13.1.0]
Rehearsal ----------------------------------------------------
http               0.030000   0.010000   0.040000 (  2.524550)
http+persistent    0.010000   0.000000   0.010000 (  2.543660)
https+persistent   0.020000   0.000000   0.020000 (  1.641750)
https              0.020000   0.010000   0.030000 (  4.863241)
------------------------------------------- total: 0.100000sec

                       user     system      total        real
http               0.010000   0.000000   0.010000 (  2.866540)
http+persistent    0.010000   0.000000   0.010000 (  2.299575)
https+persistent   0.010000   0.010000   0.020000 (  1.864620)
https              0.030000   0.000000   0.030000 (  5.646562)