Support For Text Search

Currently Supported Operations:

1) Text search on boolean, number, and strings
2) Text Search on sub fields and arrays
3) Default Search when no fields are provided
4) Pagination via $bookmark operator
5) Faceted Search via $counts and $ranges operators
6) Sort 
7) Use of analyzers

Not Currently Supported:
1) Group By 
2) Sort by location

Examples are in tests folder.

Need more negative tests.

Open Discussion:
If the user doesn't provide any fields, then we use the default field which indexes all values from the fields that were provided during text creation. Hence, if an index has "fields":["age","name"],
then the default field would index both "age" and "name". 
When multiple indexes exist, then each index will have a "default" field. If the user searches without providing a field name, then which index do we choose? Currently we choose the index contains the most fields. 

In general, multiple indexes pose a problem for selection. Right now, we allow different indexes to
index the same set of fields. These indexes, however, could have different options for each field. 
Which index should we choose? Currently, we choose the index created last. 
Add a mango_idx API function for compatible sorts. This logic shouldn't be outside the individual modules.