DocumentService - Add new search method

[pborissow]

Implement a new method to search a lucene-based index for documents that contain a search term in the body of the document or in the file name itself. Search results should be sorted by relevance and return file info (e.g. file name, size, date, etc), a search metadata (e.g. relevance score, number of occurrences, etc)

[pborissow]

Although this was largely implemented in #216 there's room for improvement. For example:

• Support for search terms in quotes (e.g. "White Flint")
• Including a sentence fragment from the document that includes the search term
• Number of occurrences

Reference: https://stackoverflow.com/questions/1920726/get-search-word-hits-number-of-occurences-per-document-in-lucene