IPNet fix and refactoring

[Arkoniak]

1. Instead of internal IPNet implementation proper IPNets.jl is used.
2. update, readmd5 and so forth were removed since they are unusable anymore (due to the MaxMind policy).
3. load now returns local geodata object instead of updating global variable.
4. load returns GeoIP.DB structure instead of DataFrame.
5. load now supports double gz and zip file format.
6. Additional environment variable GEOIP_ZIPFILE can be used now.
7. Broadcasting is now directly supported for GeoIP.DB type.
8. docs and README updated

[codecov-commenter]

Codecov Report

Merging #49 (e8f952b) into master (06f0970) will increase coverage by 49.06%.
The diff coverage is 73.33%.

@@             Coverage Diff             @@
##           master      #49       +/-   ##
===========================================
+ Coverage   32.74%   81.81%   +49.06%     
===========================================
  Files           4        3        -1     
  Lines         171       44      -127     
===========================================
- Hits           56       36       -20     
+ Misses        115        8      -107     

Impacted Files	Coverage Δ
src/GeoIP.jl	100.00% <ø> (ø)
src/geoip-module.jl	82.35% <50.00%> (+12.78%)	⬆️
src/data.jl	80.76% <81.81%> (+49.86%)	⬆️

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