Except IOError in iter_gp and yield None instead of breaking the iter…

…ation.

Add IntervalReadingMixin for reading small files in intervals.

This also makes a small change in daily_images, making sure that a whole day
from start to finish is requested.

Also extends the tests.

Update changelog

[ci skip]
[skip ci]

Merge pull request #37 from sebhahn/fix10

Fix issue 10 and add new spatial subset method

Drop Python 3.3 support in travis.

Conda packages for 3.3 are not built.

Update Changelog.

Update changelog.

update change log

prepare for release v0.3.6

Update changelog for v0.3.5.