Add not equals validation

[jsixface]

Add Not equals validation to the where clause.

gitql "select committer from commits where committer != 'Arumugam J' order by date desc limit 1"
+-----------+
| committer |
+-----------+
| GitHub    |
+-----------+

It was originally not validating the where clause if it has a != sign. Fixes #58

[luizperes]

Hi @jsixface, is there any problem with the operator <> ? I am asking that because this is the actual lexeme for not equals in SQL, right? Thanks! :)

[jsixface]

In my SQL experience, I've never used <> for not equal to. Is that the proper usage?

[luizperes]

what do you think of supporting both? (as per https://stackoverflow.com/questions/723195/should-i-use-or-for-not-equal-in-t-sql)

[jsixface]

yeah. I'll add support for <> as well.

[jsixface]

@luizperes Added the operator <> for not equal to.

[luizperes]

Sounds good @jsixface! Thanks for your contributions and sorry for the delay in reviewing it. :)