Fix integrate for several variables and add simple tests

[dpsanders]

Fixes #146.

[coveralls]

Coverage decreased (-0.002%) to 97.183% when pulling 237632b on fix_integrate into 4a1eed0 on master.

[dpsanders]

Tests pass on 0.6.
Something is going wrong with allowing failures on 0.7.

Shall we merge anyway?

[dpsanders]

Apparently I didn't have latest master with latest .travis.yml.
Have now rebased against latest master which should hopefully fix the problem.

[dpsanders]

@lbenet This is ready to merge.

[lbenet]

Thanks a lot! I'll merge this!

[lbenet]

Incidentally, the problem with some tests for Julia 0.7 seems to be related to the need of loading certain parts which in 0.7 are the stdlib. I have done already something on this, without caring about 0.6 compatibility; see the branch deprecations0.7.