Make it faster

[madeso]

We can't test how fast/slow this is, so some form of test is absolutely needed to compare to other json parsers and most importantly so previous builds (#26)

That being said, I guess these are some candidates for slowness:

• the "many" shared_ptr allocations Replace shared_ptr #25
• the horrible but easy to read and modify parser
• the fact that there is no streaming api (by design) forcing us to read a whole file into memory before we can parse (and allocate the strings again)