yajl_tree enhancements

[nkhorman]

I think these changes would be generally useful for others, and would like to see them merged please.

Rework yajl_tree_parse to add support for other content retrieval sources. This, I think, implements the TODO 'investigate pull instead of push parsing' by using a common read() function and a callback to obtain more content to feed to the parser.

Add a simple Json Path API.

[Alessar]

I think this pull request contains very usefull improvements for yajl library. Yajl needs such changes. The main feature which is situated in this PR is possibility to read file and create full tree of values. In current state of yajl library every users should do

• Parse file by chunks using low-level api
• Check that content has no errors
• Allocate large string and copy full content of file into that string
• Call yajl_tree_parse and get full tree of json values.

After the changes suggested by this PR the same can be done by just

• Invoke yajl_tree_parse_file. It returns correct tree of json values immediately

I think it can be very important for everybody who use yajl library. And YAJL will become more user friendly than it is now. Let's make YAJL better together!