Add support for case-intensive search

The current search() implementation performs a case-sensitive match. We should add a flag to the search call to make it case insensitive, i.e., 

``` scala
def search(query: String, caseInsensitive: Boolean = false): RDD[Long]
```

For a query string `abc`, it would be equivalent to executing the following regular expression query:

```
[aA][bB][cC]
```
