Don't treat symbols as words

This change ruined the navigation via M-f/M-b because it would make the
navigation on a variable like:

  one_two_three

jump across the whole word, instead of moving up to the '_' within the
word.
Highlight properly variables with underscores

Quick fix for variable highlighting when they do contain underscores in
their name
Added tests and imenu syntax fix for '_'

Added test to check for the correct navigation for variables which
contain underscores.
Also added '_' to the imenu regexp.
Merge pull request #266 from dotemacs/variable-name-fix

Variable name fix