Feature: Support ujson if installed

[danigosa]

If installed but no wish to use it set HUG_USE_UJSON=0 environmental variable. This is for the sake of people not wanting to use ujson even when installed in the python environment.

This would increase the performance dramatically for those microservices that are basically JSON loaders/dumpers endpoints.

[coveralls]

Coverage decreased (-0.04%) to 99.806% when pulling 366a15a on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[coveralls]

Coverage decreased (-0.3%) to 99.535% when pulling 73c2307 on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[coveralls]

Coverage decreased (-0.3%) to 99.535% when pulling f78b5e1 on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[danigosa]

@timothycrosley using it in our internal projects successfully

[coveralls]

Coverage decreased (-0.3%) to 99.496% when pulling da1785d on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[coveralls]

Coverage decreased (-0.4%) to 99.458% when pulling d995da4 on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[coveralls]

Coverage decreased (-0.4%) to 99.458% when pulling d995da4 on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[coveralls]

Coverage increased (+0.0001%) to 99.844% when pulling b26c952 on danigosa:feature/ujson_support into aec8a6f on timothycrosley:develop.

[timothycrosley]

Looks good, thanks!