Make JSON support optional

[bog-dan-ro]

Close #215

[msteinbeck]

Hi @bog-dan-ro,

thank you for your contributions. I would like to avoid adding new compilation options to TinySpline because this makes it necessary to test more combinations of options. Moreover, if JSON serialization is disabled, certain tests must also be disabled. Rather than conditionally include/exclude features, I would prefer to rename the affected functions. In the long run, I would like to replace parson with a custom implementation anyways (I full fledged JSON library is not needed to write splines as JSON; parsing might be more difficult though).