remove requests for smaller memory usage

[remorses]

I like the fact that this library is very fast compared to json schema, but in term of memory usage this lib isn't any better.
The problem Is that requests takes about 15 Mb of memory after being loaded and the result is that the module is 20 Mb big.
Maybe I will find time to replace requests with urllib, are you ok with a pull request like that?
memory_profiler result:

Line #    Mem usage    Increment   Line Contents
================================================
     6     24.7 MiB     24.7 MiB       @profile
     7                                 def main():
     8
     9     40.5 MiB     15.8 MiB           import fastjsonschema

The lib would takes 0.1 Mb of memory without requests and about 6 Mb using urllib

Line #    Mem usage    Increment   Line Contents
================================================
     6     24.7 MiB     24.7 MiB       @profile
     7                                 def main():
     8
     9     40.4 MiB     15.7 MiB           import requests

    10     40.5 MiB      0.1 MiB           import fastjsonschema

[horejsek]

That's interesting. Yes, I would allow it. There is only one call in ref_resolve, so it should be quite easy. Maybe would be good to write the test to check memory usage so in the future it will stay reasonable as well. :-)

[remorses]

Made pull request #44.