Fix the default ordering of dicts and add support for sorting keys

[sdispater]

Resolves #93

This PR fixes an issue where the order of the original dict was not preserved when dumping it.

It also adds the ability to tell TOMLKit to sort keys when dumping by passing sort_keys=True to dumps().

[codecov-commenter]

Codecov Report

Merging #103 into master will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   91.80%   91.71%   -0.09%     
==========================================
  Files          11       11              
  Lines        2391     2391              
==========================================
- Hits         2195     2193       -2     
- Misses        196      198       +2     

Impacted Files	Coverage Δ
tomlkit/api.py	98.71% <100.00%> (ø)
tomlkit/items.py	88.14% <100.00%> (ø)
tomlkit/parser.py	96.26% <0.00%> (-0.28%)	⬇️

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