Fix escaping in JSON pointers

[grktsh]

#13 refers old draft specification. RFC 6901 says:

   Because the characters '~' (%x7E) and '/' (%x2F) have special
   meanings in JSON Pointer, '~' needs to be encoded as '~0' and '/'
   needs to be encoded as '~1' when these characters appear in a
   reference token.

[coveralls]

Coverage decreased (-0.4%) to 91.148% when pulling 1542c82 on grktsh:fix-escape into 066132e on gazpachoking:master.

[IdanHo]

@gazpachoking Hey, can this be reviewed/merged? This change is required in order to parse valid json pointers as per the specification.

[gazpachoking]

Thanks!

[IdanHo]

Thanks grktsh, and thanks gazpachoking for the merge.