Fix typo in order_by() syntax in documentation

It was using backticks instead of single quotes.
Sanitize input to offset() and limit()
Coding standards fixes
Incorrect usage of UNION and ORDER BY without parenthesis

When use ORDER BY with UNION, give error Incorrect usage of UNION and ORDER BY. http://stackoverflow.com/a/6732697
Add travis build badges

[skip ci]
Merge pull request #42 from cmwelsh

Fix typo in order_by() syntax in documentation.

Merging manually as the original contribution was targeted
to 3.3/master.
Merge pull request #48 from @jzskca

Sanitize input to offset() and limit()

Merged manually as the original PR was to 3.3/master.

Closes #48.
Merge pull request #55 from misterx:patch-1

Incorrect usage of UNION and ORDER BY without parenthesis

Applied manually as the original PR was targeted to 3.3/master.

Fixes #55