Handle case where data to be highlighted is None, and minor refactoring

[ajiteshr7]

1. Handle case where data of the field to be highlighted is None,
   causing highlight method to fail since it expects data to be of string type. Fixes Highlighting fails with TypeError when highlighter_field value is None (while using HighlighterMixin) #155

2. Taking out the extraction of terms from the request into a method of its own,
   namely get_terms so that its easier to override it with a custom implementation,
   adds to separation of concerns.

[ajiteshr7]

There's one more issue with this highlighter, since it get the terms to highlight from the request data.
It actually also highlights page number present in the text, since when ?page=<page_number> is present in the query.
@rhblind

[coveralls]

Coverage increased (+0.07%) to 88.606% when pulling ba350e0 on ajiteshr7:master into bb5bdb5 on rhblind:master.

[coveralls]

Coverage decreased (-0.5%) to 88.006% when pulling ba350e0 on ajiteshr7:master into bb5bdb5 on rhblind:master.

[rhblind]

Beautiful, thank you very much!